## Multiple-junction quantum cascade photodetectors for thermophotovoltaic energy conversion

Optics Express, Vol. 18, Issue 2, pp. 1618-1629 (2010)

http://dx.doi.org/10.1364/OE.18.001618

Acrobat PDF (307 KB)

### Abstract

The use of intersubband transitions in quantum cascade structures for thermophotovoltaic energy conversion is investigated numerically. The intrinsic cascading scheme, spectral agility, and design flexibility of these structures make them ideally suited to the development of high efficiency multiple-junction thermophotovoltaic detectors. A specific implementation of this device concept is designed, based on bound-to-continuum intersubband transitions in large-conduction-band-offset In_{0.7}Ga_{0.3}As/AlAs_{0.8}Sb_{0.2} quantum wells. The device electrical characteristics in the presence of thermal radiation from a blackbody source at 1300 K are calculated, from which a maximum extracted power density of 1.4 W/cm^{2} is determined. This value compares favorably with the present state-of-the-art in interband thermophotovoltaic energy conversion, indicating that quantum cascade photodetectors may provide a promising approach to improve energy extraction from thermal sources.

© 2010 OSA

## 1. Introduction

2. P. F. Baldasaro, J. E. Raynolds, G. W. Charache, D. M. DePoy, C. T. Ballinger, T. Donovan, and J. M. Borrego, “Thermodynamic analysis of thermophotovoltaic efficiency and power density tradeoffs,” J. Appl. Phys. **89**(6), 3319–3327 (2001). [CrossRef]

8. L. Bhusal and A. Freundlich, “GaAsN/InAsN superlattice based multijunction thermophotovoltaic devices,” J. Appl. Phys. **102**(7), 074907 (2007). [CrossRef]

10. M. Graf, G. Scalari, D. Hofstetter, J. Faist, H. Beere, E. Linfield, D. Ritchie, and G. Davies, “Terahertz range quantum well infrared photodetectors,” Appl. Phys. Lett. **84**(4), 475–477 (2004). [CrossRef]

15. D. Hofstetter, F. R. Giorgetta, E. Baumann, Q. Yang, C. Manz, and K. Köhler, “Midinfrared quantum cascade detector with a spectrally broad response,” Appl. Phys. Lett. **93**(22), 221106 (2008). [CrossRef]

15. D. Hofstetter, F. R. Giorgetta, E. Baumann, Q. Yang, C. Manz, and K. Köhler, “Midinfrared quantum cascade detector with a spectrally broad response,” Appl. Phys. Lett. **93**(22), 221106 (2008). [CrossRef]

## 2. Device design

_{0.7}Ga

_{0.3}As/ AlAs

_{0.8}Sb

_{0.2}QWs, which can be grown strain compensated on InP substrates and which have been used recently to demonstrate room-temperature quantum-cascade laser emission down to about 3.1 μm [16

16. S. Y. Zhang, D. G. Revin, J. W. Cockburn, K. Kennedy, A. B. Krysa, and M. Hopkinson, “λ~3.1 μm room temperature InGaAs/AlAsSb/InP quantum cascade lasers,” Appl. Phys. Lett. **94**(3), 031106 (2009). [CrossRef]

17. I. Vurgaftman, J. R. Meyer, and L. R. Ram-Mohan, “Band parameters for III-V compound semiconductors and their alloys,” J. Appl. Phys. **89**(11), 5815–5875 (2001). [CrossRef]

_{0.7}Ga

_{0.3}As/AlAs

_{0.8}Sb

_{0.2}multiple QWs grown on InP, the In

_{0.7}Ga

_{0.3}As well layers have conduction-band minimum in the Γ valley, and the Γ-point conduction-band offset is as large as ~1.5 eV [16

16. S. Y. Zhang, D. G. Revin, J. W. Cockburn, K. Kennedy, A. B. Krysa, and M. Hopkinson, “λ~3.1 μm room temperature InGaAs/AlAsSb/InP quantum cascade lasers,” Appl. Phys. Lett. **94**(3), 031106 (2009). [CrossRef]

_{0.8}Sb

_{0.2}barrier layers feature an indirect energy bandgap with conduction-band minima in the X valleys. The effective conduction-band offset

*ΔE*of these QWs is therefore determined by the energy difference between the X-valley minima

_{c}*E*of the barriers and the Γ-valley minimum

_{X}*E*of the wells, which is estimated to be about 760 meV. Γ-valley confined states with energy above this effective barrier are also supported by the In

_{Γ}_{0.7}Ga

_{0.3}As wells; however, such states can be expected to be very short lived due to fast leakage into the nearby unbound X-valley states, leading to additional dark current in devices such as the exemplary structure of Fig. 1. Thus, all QCP structures in this work are designed so that their operation only involves Γ-valley bound states with energy below the effective Γ-to-X barrier height

*ΔE*. As discussed in the following, this restriction provides the main performance limiting factor of the multiple-junction TPV device modeled in the present study.

_{c}_{0.8}Sb

_{0.2}barriers and the underlined numbers indicate the

*n*-type doped layers, with the doping density specified in multiples of 10

^{18}cm

^{−3}by the subsequent number in parentheses. The structure shown in Fig. 1 is based on the design labeled QCP2 in the table.

*ΔE*. Higher conversion efficiencies could therefore be obtained using QWs with even larger conduction-band offsets, which can be realized for example with III-nitride or II-VI semiconductors. However the ISB device technology of the latter materials systems is still in a relatively early stage, which is why in the present work we consider the more mature family of InGaAs/AlAsSb QWs. Alternatively, the QCP stages of Table 2 could be further integrated with a photovoltaic

_{c}*p-n*junction device providing interband absorption at shorter wavelengths. These considerations are discussed in more detail in Section 4 below.

19. J. Y. Andersson and L. Lundqvist, “Near-unity quantum efficiency of AlGaAs/GaAs quantum well infrared detectors using a waveguide with a doubly periodic grating coupler,” Appl. Phys. Lett. **59**(7), 857–859 (1991). [CrossRef]

## 3. Simulation method

19. J. Y. Andersson and L. Lundqvist, “Near-unity quantum efficiency of AlGaAs/GaAs quantum well infrared detectors using a waveguide with a doubly periodic grating coupler,” Appl. Phys. Lett. **59**(7), 857–859 (1991). [CrossRef]

*A*and

*B*. Due to the strong coupling among states within each cascade, intra-cascade thermalization is much faster than inter-cascade scattering. As a result, all electrons within each cascade can be assumed to be in a state of quasi-thermal equilibrium with one another, described by a quasi-Fermi level which in the presence of photovoltage is different for the different cascades. In this respect the physics of QCP structures is therefore analogous to that of traditional

*p-n*junctions where the valence and conduction bands similarly support independent thermal distributions [12

12. C. Koeniguer, G. Dubois, A. Gomez, and V. Berger, “Electronic transport in quantum cascade structures at equilibrium,” Phys. Rev. B **74**(23), 235325 (2006). [CrossRef]

20. R. Köhler, R. C. Iotti, A. Tredicucci, and F. Rossi, “Design and simulation of terahertz quantum cascade lasers,” Appl. Phys. Lett. **79**(24), 3920–3922 (2001). [CrossRef]

*α*is the dimensionless absorption coefficient normalized to the stage inverse thickness 1/

_{2D}*t*;

_{st}*i*and

*j*run over all states of two consecutive cascades

*A*and

*B*, respectively; the energy at the bottom of subband

*i*(

*E*) is computed by solving the Schrödinger equation for the given QCP structure; the dipole moment matrix element between subbands

_{i}*i*and

*j*(

*qz*) is calculated from their respective envelope functions; the absorption full width at half maximum of the

_{ij}*i*-to-

*j*transition (Γ

*) is taken to be a fixed fraction (5%) of the transition energy*

_{ij}*E*–

_{j}*E*, in accordance with typical measurements from similar QWs [21]; finally, the electron sheet density of subband

_{i}*i*(

*N*) and the corresponding quasi-Fermi energy

_{i}*N*electrons to all subbands of cascade

_{D}*A*according to a quasi-Fermi distribution (

*N*being the donor sheet density per stage). Throughout this work, the device temperature

_{D}*T*is taken to be 300 K.

*α*of 3-5 × 10

_{2D}^{−3}per stage are observed. If these QCP structures are embedded in a planar waveguide with a typical guided-mode thickness

*t*of about 1 μm and lateral dimensions

_{wg}*L*of several millimeters, the corresponding modal absorbance (

*α*/

_{2D}*t*)(

_{st}*t*/

_{st}*t*)

_{wg}*L*has peak values of about 10 or larger per stage. In this case, a single repetition of each QCP structure within the waveguide core is therefore sufficient to provide nearly complete absorption of the in-coupled light over the structure absorption bandwidth. The simulations presented in this article refer to such a situation. Alternatively, one could use

*N*repetitions of each QCP structure grown on top of one another, resulting in an

*N*-fold decrease in photocurrent compensated by a proportional increase in cumulative photovoltage.

*T*(1300 K in the present case). The integration range is the absorption bandwidth (

_{rad}*BW*) of the given stage, which here is defined as the spectral region over which the stage absorption coefficient

*α*[as determined from Eq. (1)] is larger than 2 × 10

_{2D}^{−3}. While the specific choice of this threshold value is somewhat arbitrary, we notice that it is large enough to justify the assumption of complete absorption with a single stage. Furthermore, the relatively steep edges of the calculated absorption spectra make the integration limits of Eq. (2) rather insensitive to the specific choice of threshold. The calculated photocurrent densities are found to exhibit a weak but finite dependence on the voltage across the stage, which is due to the changes in subband energies and envelope functions, and therefore in absorption spectra, with varying voltage. We also note that by design there is no overlap among the absorption bandwidths

*BW*of the four structures of Table 2.

12. C. Koeniguer, G. Dubois, A. Gomez, and V. Berger, “Electronic transport in quantum cascade structures at equilibrium,” Phys. Rev. B **74**(23), 235325 (2006). [CrossRef]

*A*and

*B*via LO-phonon scattering. While all possible pairings of initial and final subbands are included in the simulations, only a few give an appreciable contribution based on the interplay between their mutual envelope-function overlap and the occupation probability of the initial state. The resulting dark current density

*J*can be written as a function of voltage

_{d}*V*in the form of a diode equationwhere the reverse saturation current density

*J*is given byReferring to Fig. 1, the quantities

_{0}*J*and

_{d}*V*introduced in these equations are defined to be positive when electrons flow from left to right and when the conduction-band edge increases from right to left, respectively. The exponential increase in dark current with increasing voltage in Eq. (3) is due to the resulting redistribution of the electrons within each cascade into states of higher and higher energy, which have a larger and larger overlap with the states of the adjacent cascade downstream. In Eq. (4),

*m** and

*ћω*are the electronic effective mass and the LO-phonon energy of the QW material;

_{LO}*T*of 300 K;

*i*in cascade

*A*;

*ε*in subband

*i*to an initially empty state in subband

*j*via LO-phonon absorption and emission, respectively. The latter quantities depend on the subband energies and envelope functions, and are computed following Ref. 22.

## 4. Results and discussion

*J*≡

_{tot}*J*–

_{p}*J*as a function of photovoltage

_{d}*V*for each QCP structure of Table 2. The resulting electrical characteristics are shown in Fig. 4(a) . For simplicity, each trace here was computed by considering two adjacent cascades based on the same design, as in the example of Fig. 1. If only one repetition of each structure is used in the multiple-junction device, the staircase of bound states funneling electrons from each stage to the next should be properly modified. In any case, these states play a negligible role in the calculated dark and photo-current densities so that their detailed energies and envelope functions are not important. We also note that the observed fluctuations in

*J*with

_{tot}*V*(superimposed to the expected diode-like exponential behavior) are due to the voltage dependence of the QW subband structure, and hence of

*α*,

_{2D}*J*. The resulting plots of output power density versus photovoltage are shown in Fig. 4(b). The operating points where these extracted powers are maxima are indicated by the symbols in Fig. 4, and can be seen to occur at roughly the same current density of about 4.2 A/cm

_{tot}V^{2}for all four stage architectures. As already mentioned, this is the result of carefully tailoring the ISB absorption spectra of Fig. 2, and ensures optimal performance of the combined TPV device comprising the series combination of the four QCP structures. Specifically, in this case the maximum output power of the combined device is as large as the sum of the maximum output powers of its individual stages, which adds up to about 1.4 W/cm

^{2}.

*J*, starting from exponential fits to the data of Fig. 4(a). Incidentally it should be noted that in general the electric fields present in the four cascaded structures at any operating point of the combined device are different due to their different electrical characteristics. The solid line in Fig. 5 is the corresponding extracted power density

_{tot}*P = J*plotted versus the combined photovoltage

_{tot}V_{tot}*V*. The short-circuit current density, open-circuit voltage, and fill factor obtained from this figure are 4.9 A/cm

_{tot}^{2}, 0.48 V, and 59%, respectively; the maximum extracted power density is 1.4 W/cm

^{2}as expected.

^{2}, obtained with a 0.6-eV-bandgap-energy InGaAs interband photodetector grown lattice mismatched on InP with an InPAs buffer [5

5. B. Wernsman, R. R. Siergiej, S. D. Link, R. G. Mahorter, M. N. Palmisiano, R. J. Wehrer, R. W. Schultz, G. P. Schmuck, R. L. Messham, S. Murray, C. S. Murray, F. Newman, D. Taylor, D. M. DePoy, and T. Rahmlow, “Greater than 20% radiant heat conversion efficiency of a thermophotovoltaic radiator/module system using reflective spectral control,” IEEE Trans. Electron. Dev. **51**(3), 512–515 (2004). [CrossRef]

3. L. M. Fraas, J. E. Avery, H. X. Huang, and R. U. Martinelli, “Thermophotovoltaic system configurations and spectral control,” Semicond. Sci. Technol. **18**(5), S165–S173 (2003). [CrossRef]

6. R. R. Siergiej, S. Sinharoy, T. Valko, R. J. Wehrer, B. Wernsman, S. D. Link, R. W. Schultz, and R. L. Messham, “InGaAsP/InGaAs tandem TPV device,” AIP Conf. Proc. **738**, 480–488 (2004). [CrossRef]

8. L. Bhusal and A. Freundlich, “GaAsN/InAsN superlattice based multijunction thermophotovoltaic devices,” J. Appl. Phys. **102**(7), 074907 (2007). [CrossRef]

^{2}from a thermal radiator at 1350 K was computed. The results presented in Figs. 4 and 5 correspond to a lower blackbody-source temperature of 1300 K; by repeating these calculations for the same multiple-junction QCP device in the presence of 1350-K thermal radiation we obtain a maximum output power of over 1.6 W/cm

^{2}, which again represents a substantial improvement.

_{0.7}Ga

_{0.3}As/ AlAs

_{0.8}Sb

_{0.2}device modeled in this work is not necessarily optimized in terms of number of stages and spectral coverage, and it is possible that different configurations (still based on the same materials system) may provide larger values of the extracted power. Even greater improvements can be expected if QW systems with larger conduction-band offsets are employed, so that a larger fraction of the incident radiation spectrum can be absorbed. Two attractive candidates in this respect are GaN/Al(Ga)N and ZnCdSe/(Zn)(Cd)MgSe QWs, where the usable barrier height can be as large as 1.75 and 1.2 eV, respectively. While neither materials system has yet reached the same level of maturity for ISB device development as As-based QWs, significant advances have been reported recently, including the demonstration of QCP detection with GaN at the record short wavelength of 1.7 μm [14

14. A. Vardi, G. Bahir, F. Guillot, C. Bougerol, E. Monroy, S. E. Schacham, M. Tchernycheva, and F. H. Julien, “Near infrared quantum cascade detector in GaN/AlGaN/AlN heterostructures,” Appl. Phys. Lett. **92**(1), 011112 (2008). [CrossRef]

23. K. J. Franz, W. O. Charles, A. Shen, A. J. Hoffman, M. C. Tamargo, and C. Gmachl, “ZnCdSe/ZnCdMgSe quantum cascade electroluminescence,” Appl. Phys. Lett. **92**(12), 121105 (2008). [CrossRef]

*p-n*junction photovoltaic detector operating at shorter wavelengths. This

*p-n*junction may be grown directly over the

*n*-type multiple-stage QCP structure, which should then be oriented so as to produce a photocurrent flowing towards the top contact. A particularly convenient material choice for the additional

*n*and

*p*layers is In

_{0.53}Ga

_{0.47}As, which is lattice matched to InP and to strain-compensated In

_{0.7}Ga

_{0.3}As/AlAs

_{0.8}Sb

_{0.2}QWs, and whose bandgap energy of 0.74 eV is above the high-energy end of the combined absorption spectrum of Fig. 2. In fact, such a

*p-n*junction could be employed by itself for TPV energy conversion with reasonable performance [2

2. P. F. Baldasaro, J. E. Raynolds, G. W. Charache, D. M. DePoy, C. T. Ballinger, T. Donovan, and J. M. Borrego, “Thermodynamic analysis of thermophotovoltaic efficiency and power density tradeoffs,” J. Appl. Phys. **89**(6), 3319–3327 (2001). [CrossRef]

## 5. Conclusions

_{0.7}Ga

_{0.3}As/AlAs

_{0.8}Sb

_{0.2}QWs, which feature a relatively large conduction-band offset and a well established ISB device technology. An exemplary implementation of a multiple-junction QCP comprising four different stage architectures was designed based on these QWs. The device electrical characteristics were then calculated under the assumptions of unit quantum efficiency throughout the combined absorption bandwidth and of thermal equilibrium within each cascade of highly connected electronic states. Correspondingly, a maximum output power density of 1.4 W/cm

^{2}in the presence of thermal radiation from a blackbody at 1300 K was computed. This limiting value compares favorably with the present state-of-the-art in TPV energy extraction from a similar thermal source (about 0.8 W/cm

^{2}as reported in Ref. 5

5. B. Wernsman, R. R. Siergiej, S. D. Link, R. G. Mahorter, M. N. Palmisiano, R. J. Wehrer, R. W. Schultz, G. P. Schmuck, R. L. Messham, S. Murray, C. S. Murray, F. Newman, D. Taylor, D. M. DePoy, and T. Rahmlow, “Greater than 20% radiant heat conversion efficiency of a thermophotovoltaic radiator/module system using reflective spectral control,” IEEE Trans. Electron. Dev. **51**(3), 512–515 (2004). [CrossRef]

*p*-

*n*junction devices, may provide a promising new approach to improve energy extraction efficiency from thermal sources.

## Acknowledgements

## References and links

1. | R. Paiella, ed., |

2. | P. F. Baldasaro, J. E. Raynolds, G. W. Charache, D. M. DePoy, C. T. Ballinger, T. Donovan, and J. M. Borrego, “Thermodynamic analysis of thermophotovoltaic efficiency and power density tradeoffs,” J. Appl. Phys. |

3. | L. M. Fraas, J. E. Avery, H. X. Huang, and R. U. Martinelli, “Thermophotovoltaic system configurations and spectral control,” Semicond. Sci. Technol. |

4. | K. Emery, “Characterizing thermophotovoltaic cells,” Semicond. Sci. Technol. |

5. | B. Wernsman, R. R. Siergiej, S. D. Link, R. G. Mahorter, M. N. Palmisiano, R. J. Wehrer, R. W. Schultz, G. P. Schmuck, R. L. Messham, S. Murray, C. S. Murray, F. Newman, D. Taylor, D. M. DePoy, and T. Rahmlow, “Greater than 20% radiant heat conversion efficiency of a thermophotovoltaic radiator/module system using reflective spectral control,” IEEE Trans. Electron. Dev. |

6. | R. R. Siergiej, S. Sinharoy, T. Valko, R. J. Wehrer, B. Wernsman, S. D. Link, R. W. Schultz, and R. L. Messham, “InGaAsP/InGaAs tandem TPV device,” AIP Conf. Proc. |

7. | M. W. Dashiell, J. F. Beausang, H. Ehsani, G. J. Nichols, D. M. DePoy, L. R. Danielson, P. Talamo, K. D. Rahner, E. J. Brown, S. R. Burger, P. M. Fourspring, W. F. Topper Jr, P. F. Baldasaro, C. A. Wang, R. K. Huang, M. K. Connors, G. W. Turner, Z. A. Shellenbarger, G. Taylor, J. Li, R. Martinelli, D. Donetski, S. Anikeev, G. L. Belenki, and S. Luryi, “Quaternary InGaAsSb thermophotovoltaic diodes,” IEEE Trans. Electron. Dev. |

8. | L. Bhusal and A. Freundlich, “GaAsN/InAsN superlattice based multijunction thermophotovoltaic devices,” J. Appl. Phys. |

9. | M. A. Green, |

10. | M. Graf, G. Scalari, D. Hofstetter, J. Faist, H. Beere, E. Linfield, D. Ritchie, and G. Davies, “Terahertz range quantum well infrared photodetectors,” Appl. Phys. Lett. |

11. | L. Gendron, M. Carras, A. Huynh, V. Ortiz, C. Koeniguer, and V. Berger, “Quantum cascade photodetector,” Appl. Phys. Lett. |

12. | C. Koeniguer, G. Dubois, A. Gomez, and V. Berger, “Electronic transport in quantum cascade structures at equilibrium,” Phys. Rev. B |

13. | F. R. Giorgetta, E. Baumann, D. Hofstetter, C. Manz, Q. Yang, K. Köhler, and M. Graf, “InGaAs/AlAsSb quantum cascade detectors operating in the near infrared,” Appl. Phys. Lett. |

14. | A. Vardi, G. Bahir, F. Guillot, C. Bougerol, E. Monroy, S. E. Schacham, M. Tchernycheva, and F. H. Julien, “Near infrared quantum cascade detector in GaN/AlGaN/AlN heterostructures,” Appl. Phys. Lett. |

15. | D. Hofstetter, F. R. Giorgetta, E. Baumann, Q. Yang, C. Manz, and K. Köhler, “Midinfrared quantum cascade detector with a spectrally broad response,” Appl. Phys. Lett. |

16. | S. Y. Zhang, D. G. Revin, J. W. Cockburn, K. Kennedy, A. B. Krysa, and M. Hopkinson, “λ~3.1 μm room temperature InGaAs/AlAsSb/InP quantum cascade lasers,” Appl. Phys. Lett. |

17. | I. Vurgaftman, J. R. Meyer, and L. R. Ram-Mohan, “Band parameters for III-V compound semiconductors and their alloys,” J. Appl. Phys. |

18. | O. Madelung, ed., |

19. | J. Y. Andersson and L. Lundqvist, “Near-unity quantum efficiency of AlGaAs/GaAs quantum well infrared detectors using a waveguide with a doubly periodic grating coupler,” Appl. Phys. Lett. |

20. | R. Köhler, R. C. Iotti, A. Tredicucci, and F. Rossi, “Design and simulation of terahertz quantum cascade lasers,” Appl. Phys. Lett. |

21. | M. Helm, “The basic physics of intersubband transitions,” in |

22. | P. Harrison, |

23. | K. J. Franz, W. O. Charles, A. Shen, A. J. Hoffman, M. C. Tamargo, and C. Gmachl, “ZnCdSe/ZnCdMgSe quantum cascade electroluminescence,” Appl. Phys. Lett. |

**OCIS Codes**

(040.5350) Detectors : Photovoltaic

(250.5590) Optoelectronics : Quantum-well, -wire and -dot devices

**ToC Category:**

Detectors

**History**

Original Manuscript: December 11, 2009

Revised Manuscript: January 8, 2010

Manuscript Accepted: January 9, 2010

Published: January 13, 2010

**Virtual Issues**

Focus Issue: Solar Concentrators (2010) *Optics Express*

**Citation**

Jian Yin and Roberto Paiella, "Multiple-junction quantum cascade photodetectors for thermophotovoltaic energy conversion," Opt. Express **18**, 1618-1629 (2010)

http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-2-1618

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### References

- R. Paiella, ed., Intersubband Transitions in Quantum Structures (McGraw-Hill, New York, 2006).
- P. F. Baldasaro, J. E. Raynolds, G. W. Charache, D. M. DePoy, C. T. Ballinger, T. Donovan, and J. M. Borrego, “Thermodynamic analysis of thermophotovoltaic efficiency and power density tradeoffs,” J. Appl. Phys. 89(6), 3319–3327 (2001). [CrossRef]
- L. M. Fraas, J. E. Avery, H. X. Huang, and R. U. Martinelli, “Thermophotovoltaic system configurations and spectral control,” Semicond. Sci. Technol. 18(5), S165–S173 (2003). [CrossRef]
- K. Emery, “Characterizing thermophotovoltaic cells,” Semicond. Sci. Technol. 18(5), S228–S231 (2003). [CrossRef]
- B. Wernsman, R. R. Siergiej, S. D. Link, R. G. Mahorter, M. N. Palmisiano, R. J. Wehrer, R. W. Schultz, G. P. Schmuck, R. L. Messham, S. Murray, C. S. Murray, F. Newman, D. Taylor, D. M. DePoy, and T. Rahmlow, “Greater than 20% radiant heat conversion efficiency of a thermophotovoltaic radiator/module system using reflective spectral control,” IEEE Trans. Electron. Dev. 51(3), 512–515 (2004). [CrossRef]
- R. R. Siergiej, S. Sinharoy, T. Valko, R. J. Wehrer, B. Wernsman, S. D. Link, R. W. Schultz, and R. L. Messham, “InGaAsP/InGaAs tandem TPV device,” AIP Conf. Proc. 738, 480–488 (2004). [CrossRef]
- M. W. Dashiell, J. F. Beausang, H. Ehsani, G. J. Nichols, D. M. DePoy, L. R. Danielson, P. Talamo, K. D. Rahner, E. J. Brown, S. R. Burger, P. M. Fourspring, W. F. Topper, P. F. Baldasaro, C. A. Wang, R. K. Huang, M. K. Connors, G. W. Turner, Z. A. Shellenbarger, G. Taylor, J. Li, R. Martinelli, D. Donetski, S. Anikeev, G. L. Belenki, and S. Luryi, “Quaternary InGaAsSb thermophotovoltaic diodes,” IEEE Trans. Electron. Dev. 53(12), 2879–2891 (2006). [CrossRef]
- L. Bhusal and A. Freundlich, “GaAsN/InAsN superlattice based multijunction thermophotovoltaic devices,” J. Appl. Phys. 102(7), 074907 (2007). [CrossRef]
- M. A. Green, Third Generation Photovoltaics (Springer-Verlag, Berlin, 2006).
- M. Graf, G. Scalari, D. Hofstetter, J. Faist, H. Beere, E. Linfield, D. Ritchie, and G. Davies, “Terahertz range quantum well infrared photodetectors,” Appl. Phys. Lett. 84(4), 475–477 (2004). [CrossRef]
- L. Gendron, M. Carras, A. Huynh, V. Ortiz, C. Koeniguer, and V. Berger, “Quantum cascade photodetector,” Appl. Phys. Lett. 85(14), 2824–2826 (2004). [CrossRef]
- C. Koeniguer, G. Dubois, A. Gomez, and V. Berger, “Electronic transport in quantum cascade structures at equilibrium,” Phys. Rev. B 74(23), 235325 (2006). [CrossRef]
- F. R. Giorgetta, E. Baumann, D. Hofstetter, C. Manz, Q. Yang, K. Köhler, and M. Graf, “InGaAs/AlAsSb quantum cascade detectors operating in the near infrared,” Appl. Phys. Lett. 91(11), 111115 (2007). [CrossRef]
- A. Vardi, G. Bahir, F. Guillot, C. Bougerol, E. Monroy, S. E. Schacham, M. Tchernycheva, and F. H. Julien, “Near infrared quantum cascade detector in GaN/AlGaN/AlN heterostructures,” Appl. Phys. Lett. 92(1), 011112 (2008). [CrossRef]
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