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Hafnium nitride for hot carrier solar cells
Solar Energy Materials & Solar Cells ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Hafnium nitride for hot carrier solar cells Simon Chung a,n, Santosh Shrestha a, Xiaoming Wen a, Yu Feng a, Neeti Gupta a, Hongze Xia a, Pyng Yu b, Jau Tang b, Gavin Conibeer a a Australian Centre for Advanced Photovoltaics, School of Photovoltaics and Renewable Energy Engineering, University of New South Wales,Sydney, NSW, Australia b Research Center for Applied Sciences, Academia Sinica, Taipei, Taiwan
art ic l e i nf o
a b s t r a c t
Article history: Received 4 July 2014 Received in revised form 22 September 2014 Accepted 7 October 2014
Hot carrier solar cells is an attractive technology with the potential of reaching high energy conversion efficiencies approaching the thermodynamic limit of infinitely stacked multi-junction solar cells: 65% under one sun and 86% under maximally concentrated. The hot carrier solar cell is conceptually simple consisting of two key components: absorber and energy selective contacts. High efficiencies are achieved by minimising the energy lost to thermalisaton of hot photo-generated carriers while absorbing majority of the solar spectrum. For this to be achieved, energy selective contacts are required to allow the extraction of carriers fast enough at an energy level above the electronic band edge. It is critical for the absorber to be able to maintain a hot carrier population for a sufficiently long time period for the extraction of carriers while they are ‘hot’. Bulk materials with a large gap between acoustic and optical branches in the phonon dispersion are predicted to exhibit slow hot carrier thermalisation rates. Hafnium nitride is such a material with a large gap in its phonon dispersion and is identified as a potential material to be used as a hot carrier absorber. Hafnium nitride has been deposited using reactive sputtering and characterised to investigate material properties and carrier cooling rates. & 2014 Published by Elsevier B.V.
Keywords: Hot carrier solar cell Hafnium nitride Phonons Hot carriers Carrier cooling Transient absorption spectroscopy
1. Introduction The optimal semiconductor bandgap for a solar cell to achieve maximum energy conversion efficiency is characterised by a trade-off between energy lost to the lattice via hot carrier thermalisation and photons with energies below the bandgap not being absorbed [1]. Thermalisation occurs when hot carriers relax to the conduction band edge from elevated energy levels above the conduction band. Hot carriers refer to electrons or holes with energies above the conduction band edge or below the valance band edge. The distribution of these energetic carriers can be described by the Fermi–Dirac distribution at a higher temperature, hence the name hot carriers. Thermalisation of hot carriers in photovoltaic devices makes up a significant portion of energy lost as heat, which cannot be converted to electricity. At the optimal bandgap of 1.3 eV as described by the Shockley– Queisser limiting efficiency of p–n junction solar cells, the energy lost to thermalisation is 30% and below bandgap losses is 25% of the incident illumination [2]. The hot carrier solar cell aims to
n Correspondence to. TETB (building H6), University of New South Wales, Sydney, NSW 2052, Australia. Tel.: þ 61 4 2308 6933. E-mail address: [email protected] (S. Chung).
address these two fundamental loss mechanisms that limit the efficiency of conventional p–n junction solar cells [3–7]. The hot carrier solar cell's objective is to capture the hot carrier population of photo-generated electron hole pairs before thermalisation processes occurs. This also enables a low electronic bandgap material to be used as a photon absorber, thus minimising energy loss due to sub-bandgap photons not absorbed. The hot carriers are separated and extracted to the external circuit by the use of energy selective contacts, where only carriers of a narrow range energies elevated above the absorber's bandgap are collected. It is required to extract the carriers at a narrow range of energies to minimise the entropy generation during carrier collection, which is to prevent the cooling of hot carriers while in contact with cold carriers in the metal contact. The absorber is required to maintain a hot carrier population for a long enough time for the hot carriers to be extracted. The two key requirements of a hot carrier solar cell as shown in Fig. 1 includes an absorber with bandgap, for example, where the carrier population, n(ε), remains at elevated temperatures and energy selective contacts, ESCs where carriers are extracted with a narrow band at a high voltage. The difference in potential between the two electrodes Ve and Vh represents the open circuit voltage of such a device. We investigate the properties and carrier dynamics in hafnium nitride (HfN) in the context of an absorber layer for the Hot Carrier
http://dx.doi.org/10.1016/j.solmat.2014.10.011 0927-0248/& 2014 Published by Elsevier B.V.
Please cite this article as: S. Chung, et al., Hafnium nitride for hot carrier solar cells, Solar Energy Materials and Solar Cells (2014), http: //dx.doi.org/10.1016/j.solmat.2014.10.011i
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solar cell. While HfN's electronic properties are metallic as shown by the calculated electronic structure in Fig. 2(a) from Saha et al. [9], it has a potential to exhibit slow carrier cooling due to its phononic properties. Energy selective contacts are required to collect carriers at an elevated level above the electronic bandgap, thus it may be possible to design a hot carrier solar cell using an absorber with no electronic bandgap if the energy selective contacts are able to extract carriers at elevated energies and the hot carriers are maintained long enough in the absorber to be extracted. Materials that exhibit long hot carrier lifetimes may also be used in technologies, such as thermoelectric, plasmonic and thermionic emission devices [10–12].
2. Thermalisation of hot carriers and slowed carrier cooling In a semiconductor, electrons are excited to the conduction band from the valence band if the incident photon energy is larger than the semiconductor's bandgap. Elastic carrier–carrier scattering occurs within several femtoseconds, which redistribute the energy of the hot carrier population to a Fermi–Dirac distribution described by a temperature elevated above the lattice temperature. The hot carrier population subsequently thermalises to the band edge, which occurs from around 10 ps onwards in conventional semiconductor materials used in solar cells [13]. The hot carrier solar cells' efficiency is strongly influenced by the carrier thermalisation lifetime. For the hot carrier solar cell to exceed the Shockley–Queisser limit under 1 sun irradiation the thermalisation time is required to be around 1 ns, and under 1000 sun irradiation, it is below the limiting efficiency of triple junction cells [14].
HfN requires a carrier thermalisation time of the order of 1 ns to be suitable to be used as a hot carrier solar cell absorber, and because HfN does not contain an electronic bandgap, it would be required to be operated under highly concentrated irradiation for its efficiency to be comparable with existing multi-junction cells. The mechanism for thermalisation involves interactions between electrons and optical phonons as described by Fröhlich interactions [15]. Energetic carriers emit optical phonons that quickly decay into acoustic phonons. This is the point where the energy is lost and irretrievable to a solar cell because acoustic phonons have a large group velocity and are able to cross interfaces easily. Decay of high energy optical phonons to lower energy acoustic phonons is dominated by three particle processes known as Klemens decay and Ridley decay. These decay mechanisms adhere to conservation of energy and momentum laws. Klemens decay occurs when a phonon in the optical branch decays into two acoustic phonons with half the energy of the original phonon but each with opposite and equal momentum [16]. Ridley decay occurs when an optical phonon decays to an optical phonon of lower energy and an acoustic phonon while conserving momentum [17]. It is proposed that by preventing the decay of optical phonons to acoustic phonons, slowed carrier cooling rates can be achieved [18]. If there exist no pathway for optical phonons to decay into acoustic phonons a hot optical phonon population can be maintained, also known as a phonon bottleneck. In this situation electrons have enough time to reabsorb optical phonons hence maintaining a hot carrier population. A gap between the optical and acoustic branches of the phonon dispersion such that the minimum of the optical branch is at least twice that of the maximum of the acoustic branch can remove the dominate decay pathways, Klemens decay. Hafnium nitride is a material with a wide phonon gap because of the difference in mass between the constituent atoms and the highly symmetric rock-salt crystal structure [9]. Fig. 2(b) shows the large gap in the calculated phonon dispersion of hafnium nitride.
3. Hafnium nitride deposition
Fig. 1. Schematic and energy diagram of a hot carrier solar cell (from Takeda et al. [8]).
Thin films of HfN were deposited on Si(1 0 0) and quartz substrates using an AJA ATC-2200 magnetron sputtering system by DC sputtering of a 4 in. Hf target of 99.9% purity in a mixed argon and nitrogen atmosphere. An argon gas flow rate of 15 sccm mixed with nitrogen gas flow of 1.5 sccm was used. Before the deposition the chamber was pumped down to a base pressure of lower than 2.5 10 6 Torr. The samples were deposited with constant DC power of 50 W and at a pressure of 1.5 mTorr. Deposition occurred with the substrates heated to approximately 450 1C. The films were deposited for 1 hour. The substrates were
Fig. 2. First-principles calculations of (a) electronic structure and (b) phonon dispersionof hafnium nitride. The large gap between the acoustic and optical phonon dispersion curves is evident. Reproduced from Saha et al. [9] with permission.
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chemically cleaned using RCA1, RCA2 solutions and HF dip for silicon substrates, rinsed in DI water and blown dried with N2 just prior to loading into the load lock chamber. The Hf sputter target was sputter cleaned for, at least 10 minutes, to remove surface oxides and other impurities prior to loading the samples from the load lock to the main chamber. Prior to the film deposition, the substrates were thermally degassed at 550 1C at the base pressure of 2.5 10 6 Torr for 1 hour. Thickness of the films was measured to be approximately 45 nm with a Dektak surface profilometer. The composition of the deposited HfN films was measured using X-ray photoelectron spectroscopy, ESCALAB250Xi from Thermo Scientific with an Al Kα X-ray source. The sample was etched for 270 seconds using a 1 keV Argon ion beam to study the bulk of the HfN films. After the 270 s etch XPS estimates the bulk of the film contained 52.3% hafnium, 43.1% nitrogen and 4.6% oxygen. The chemical composition estimates were calculated by scaling the area under the respective peaks by a scaling factor. Fig. 3 shows the XPS spectra of the Hf 4 f, O 1 s and N 1 s peaks of the studied HfN sample. The locations of the Hf 4 f5/2 and 4f7/2 peaks are at 15.1 eV and 16.5 eV, respectively, which agree with work done by Perry et al. [19]. The O 1 s peak is located at 532.1 eV with a broad tail at higher energies. The cause for this broad peak near the O 1 s is unknown; possible explanations may be oxygen bonding with non-Hf impurities in the film. The binding energy of the N 1 s peak is located at 397.5 eV, which is as that for HfN reported in the literature [20]. It is presumed that the nitrogen content of the film is underestimated due to the preferential sputtering of nitrogen over hafnium atomic species. It is possible the oxygen contamination in the film occurred from oxygen impurities in the deposition chamber and oxidation of the sample surface in air. It should, however, be noted that compositional information from XPS is qualitative and method such as Rutherford Backscattering Spectroscopy is required for the accurate determination of composition. The effect of oxygen and other defects in the material is expected to decrease carrier lifetimes by introducing additional phonon modes in the phonon gap allowing decay processes to occur. A PANalytical Empyrean diffractometer with a Cu Kα X-ray source was used for ω-2θ and grazing incident X-ray diffraction (GI-XRD) scans for the analysis of the film structure. HfN films deposited on silicon exhibited a preferred growth orientation in [1 0 0] direction as evident from the ω-2θ XRD measurements, where only the HfN(2 0 0) peak is visible (see Fig. 4). GI-XRD measurements on the samples grown on Si(1 0 0) and quartz was
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done to investigate the bulk of the film and whether they are single phase. GI-XRD scans show HfN(1 1 1), HfN(2 0 0), HfN(2 2 0) and HfN(3 1 1) peaks, suggesting polycrystalline films, as seen in Fig. 5. Epitaxial growth of HfN on silicon is not expected due to a large lattice mismatch between the two materials, about 17%. In comparison to the films deposited on Si(1 0 0), the films on quartz substrates are polycrystalline with less preferential to growth in the [1 0 0] direction as higher order HfN peaks are more pronounced. Fig. 5(a) shows GI-XRD measurements of a typical HfN film deposited on a quartz substrate, with a large substrate peak at 211, HfN(1 1 1), HfN(2 0 0), HfN(2 2 0) and HfN(3 1 1) peaks. While in Fig. 5(b) for HfN film deposited on a Si(1 0 0) substrate, the HfN peaks are similar but there exists a broad peak at 301, which according to Seo et al. [21] is evidence of a nitrogen-rich phase of HfN in contrary to estimations of the film composition from XPS. From the XRD measurements there is no evidence of HfO2 peaks as the presence of oxygen is suggested by XPS. Raman spectra measurements were performed using a Renishaw inVia Raman microscope. Raman modes were excited by the 514 nm line of an Ar þ laser. As shown in Fig. 6, Raman spectroscopy results show agreement between the Raman spectrum and the calculated phonon dispersion of the HfN films. The Raman peak at 520 cm 1 is associated with zone centred
Fig. 4. ω-2θ X-ray diffraction spectrum of a hafnium nitride film deposited on Si (1 0 0) substrates. The signal intensity in the range of 251–651 has been multiplied by 100 times to facilitate comparison with the substrate, Si (4 0 0) peaks. The peak at 39.51 represents the HfN (2 0 0) peak and the small peak at 331 is an artefact of the substrate.
Fig. 3. XPS spectra of (a) Hf 4 f and (b) O 1 s and N 1 s peaks of hafnium nitride films deposited via sputtering.
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Fig. 5. Grazing incident X-ray diffraction spectrum of hafnium nitride on (a) quartz and (b) silicon substrate. The large peak 211 is that of the quartz substrate and the following peaks at 34.111, 39.571, 57.321 and 68.421 corresponds to that of (1 1 1), (2 0 0), (2 2 0) and (3 1 1) peaks of hafnium nitride. Presence of various HfN peaks demonstrates that the HfN film is polycrystalline.
Fig. 6. Raman spectra of deposited hafnium nitride film on (a) silicon substrate and (b) quartz substrate. A represents the acoustic peak and O represents the optical peak.
optical mode and Raman peak at 120–160 cm 1 can be associated with the acoustic peaks of HfN. In Fig. 6(a), for the Raman spectra of HfN deposited on Si(1 0 0) substrates, additional modes are observable, second-order acoustic modes between 230–340 cm 1, optical þ acoustic modes at 440 cm 1 and optical þacoustic modes at 650 cm 1 a, similar to the Raman spectra observed by Stoehr et al. [22] for overstoichiometric HfNx films. Although observations of first-order acoustic peaks in rock-salt structured HfN are not expected through Raman spectroscopy, the acoustic peak may be attributed to non-stoichiometric HfN polycrystalline films leading to point defects breaking the crystal symmetry leading to optical excitations of acoustic modes [22]. The large gap between the acoustic and optical Raman modes is associated with the corresponding gap in the phonon dispersion of hafnium nitride arising from the mass difference between hafnium and nitrogen atomic species. The peaks in the Raman spectrum of HfN deposited on quartz is relatively broad as shown in Fig. 6(b); the Raman spectra of samples deposited on silicon substrates displayed sharper and more defined peaks. This is an indication of the quality of the sample where a broad range of excitations in the phonon modes exist. Dominant Raman peaks associated with HfO2
are found in at 140–200 cm 1 and 500 cm 1, which also coincides with observed HfN Raman peaks [23], thus Raman scattering cannot be used to reliably distinguish the presence of HfO2 from HfN. The transmission and reflection of the HfN thin film deposited on quartz was measured using an integrating sphere with a PerkinElmer LAMBDA1050 UV/Vis/NIR spectrophotometer. From the transmission and reflection optical spectroscopy data, the absorption coefficient α of the HfN film was calculated using the following approximation, 1 T cm 1 α ln d ð1 RÞ where d is the film thickness, T is the transmission and R is the reflection of the thin film as measured by the spectrometer. Fig. 7 shows the absorption coefficient of HfN deposited on quartz. High absorption at long wavelengths is a characteristic of free electron absorption. The minimum absorption coefficient at 440 nm represents the transition point from free electron absorption and reflection to inter-band electron transitions for higher energies. It is at this point where the photo-absorbed electrons are energetic
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y = A1*exp(-x/t1) + y0
0.0035
HfN 400 nm Value
0.0030
5.69058E-4
A1
7.9332E-4
t1
0.0025
ΔOD
y0
470.55
1737.60169
0.0020 0.0015 0.0010 0.0005 0.0000
Fig. 7. Absorption coefficient, α, of hafnium nitride deposited on quartz calculated from transmission and reflection spectroscopy.
Fig. 8. The variation in the optical density spectra obtained from transient absorption spectroscopy at different probe time delays. Measurements of the hafnium nitride deposited on quartz were performed using a pump pulse of 600 mW at 400 nm.
enough to transition from N 2p-states below the Fermi level to Hf 5d-states at the Fermi level in the electronic band structure of HfN. At these higher photon energies above the electronic gap, 2.8 eV at the gamma point as shown in Fig. 2(a) strong absorption occurs.
4. Carrier lifetime measured using ultrafast transient absorption spectroscopy The femtosecond pump-probe experiments were performed on HfN samples deposited on quartz substrates with a transient absorption (TA) spectrometer (FemtoFrame II, IB Photonics). The excitation source was a 400 nm pulse of 600 mW by an OPA laser (TOPAS, Spectra Physics) with 100 fs duration and 1 kHz repetition rate. The probe beam of white light continuum was generated by focusing the 1 kHz Ti:Sapphire-amplified laser (Spitfire, Spectra Physics) into BBO crystal and detected using a polychromator-CCD. Fig. 8 shows the TA spectrum at various time delays, showing the change in optical density, ΔOD, as a function of the probe wavelength. Immediately after the ultra-short laser irradiation, electrons are excited to high excited states. Ultrafast electron– electron scattering occurs extremely within a few hundred femtoseconds. An excited state absorption peak appears at 470 nm, a bleaching band appears between 500–700 nm and broad bleaching at wavelengths greater than 700 nm. These excited absorption and bleaching peaks on these time scales suggest that the HfN sample contains many free electrons for absorption and thus there is effective electron–electron scattering between these electrons. At longer time scales from several femtoseconds up to a couple of nanoseconds a stable linear-like distribution forms in the TA
0
500
1000
1500
2000
Time delay(ps) Fig. 9. Time evolution of the variation of optical density in hafnium nitride and single exponential fitting obtained from transient absorption spectroscopy measurements.
spectrum, from 450 to 750 nm. This linear-like distribution takes several nanoseconds to relax back to the steady-state absorption levels. The linear-like distribution of the TA spectrum may possibly be representative of the tail end of a bleaching peak at a wavelength beyond the range of our data and with this tail the Fermi–Dirac distribution of electrons described by an elevated temperature. Fig. 9 presents the time evolution of the change in optical density at 470 nm with exponential fitting obtaining a decay time constant of 1.7 ns. Exponential fitting of wavelengths around the excited state absorption peak 460–490 nm and the bleaching between 710 and 740 nm yielded time constants from 0.66 ns up to 3 ns. Further analysis of the TA data is required to extract the relevant carrier temperatures and the associated carrier cooling rate. The effect of how the optical density changes in response to the laser pump would need to be modelled with respect to the material's electronic density of states. Carrier cooling rates in HfN are expected to be on similar time scales to that of the exponential fitting for the exponential fitting of these probe wavelengths. It is hypothesised that the long relaxation times are due to the large mass difference between the constituent atoms of HfN leading to a large gap between the optical and acoustic branches in the phonon dispersion. The gap in the phonon dispersion in HfN reduces carrier thermalisation rates by removing phonon decay pathways creating a phonon bottleneck effect allowing electrons to reabsorb optical phonons. Materials that exhibit extremely slow carrier thermalisation rates can be used for the development of the hot carrier solar cell, which can theoretically achieve very high power conversion efficiencies. Present hot carrier solar cell models predicts a device using an absorber material with carrier cooling rates of around 1 ns can yield efficiency greater than 50% under 1000 sun illumination. Under practical considerations the efficiency that can be achieved using HfN will be less than 50% because the model assumes an ideal case where there are no contact losses, perfect light absorption and other possible loss mechanisms. For a device to be made using HfN as an absorber energy selective contacts must be incorporated. Potential avenues for carrier selection are resonant tunnel contacts made with a thin layer between two insulating layers where resonant tunnelling can occur at a specific energy level [24].
5. Conclusion Growth of hafnium nitride thin films on silicon and quartz were done by reactive sputtering. The chemical composition of the HfN films was analysed using XPS. It showed the presence of small amounts of oxygen. For a device to be fabricated, encapsulation of
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HfN with an oxygen barrier may be required to prevent oxidation of the material, and this may also be used as a thin insulating layer for energy selective tunnel contacts. HfN deposited on Si(1 0 0) are polycrystalline and grow preferentially HfN(1 0 0) grains, while HfN deposited on quartz substrates exhibited lower crystallinity quality. Preliminary evidence from ultrafast TA spectroscopy has shown evidence of long carrier lifetimes, on the order of nanoseconds. It is worthwhile to note the quality of the samples studied can be improved, such as by deposition HfN epitaxially on MgO substrates where there is lower lattice mismatch. It is expected that epitaxial HfN will yield better results than presented here. Acknowledgements This work has been supported by the Australian government through the Australian Renewable Energy Agency (ARENA). Responsibility for the views, information or advice expressed herein is not accepted by the Australian government. References [1] W. Shockley, H.J. Queisser, Detailed balance limit of efficiency of p-n junction solar cells, J. Appl. Phys. 32 (3) (1961) 510. http://dx.doi.org/10.1063/1.1736034. [2] L.C. Hirst, N.J. Ekins-daukes, Fundamental losses in solar cells, Prog. Photovoltaics Res. Appl 19 (3) (2011) 286–293. http://dx.doi.org/10.1002/pip. [3] R. Ross, A. Nozik, Efficiency of hot-carrier solar energy converters, J. Appl. Phys. 53 (5) (1982) 3813. [4] P. Würfel, A.S. Brown, T.E. Humphrey, M.A. Green, Particle conservation in the hot-carrier solar cell, Prog. Photovoltaics Res. Appl 13 (4) (2005) 277–285. http://dx.doi.org/10.1002/pip.584. [5] M.A. Green, Third Generation Photovoltaics : Advanced Solar Energy Conversion, Springer-Verlag Berlin and Heidelberg GmbH & Co. KG, 2003. [6] P. Aliberti, Y. Feng, Y. Takeda, S.K. Shrestha, M.A. Green, G. Conibeer, Investigation of theoretical efficiency limit of hot carriers solar cells with a bulk indium nitride absorber, J. Appl. Phys. 108 (9) (2010) 094507. http://dx.doi.org/ 10.1063/1.3494047. [7] Y. Feng, P. Aliberti, B.P. Veettil, R. Patterson, S. Shrestha, M.A. Green, G. Conibeer, Non-ideal energy selective contacts and their effect on the performance of a hot carrier solar cell with an indium nitride absorber, Appl. Phys. Lett. 100 (5) (2012) 053502. http://dx.doi.org/10.1063/1.3680594. [8] Y. Takeda, T. Motohiro, D. König, P. Aliberti, Y. Feng, S. Shrestha, G. Conibeer, Practical factors lowering conversion efficiency of hot carrier solar cells, Appl. Phys. Express 3 (10) (2010) 104301. http://dx.doi.org/10.1143/APEX.3.104301.
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Hafnium nitride for hot carrier solar cells Simon Chung a,n, Santosh Shrestha a, Xiaoming Wen a, Yu Feng a, Neeti Gupta a, Hongze Xia a, Pyng Yu b, Jau Tang b, Gavin Conibeer a a Australian Centre for Advanced Photovoltaics, School of Photovoltaics and Renewable Energy Engineering, University of New South Wales,Sydney, NSW, Australia b Research Center for Applied Sciences, Academia Sinica, Taipei, Taiwan
art ic l e i nf o
a b s t r a c t
Article history: Received 4 July 2014 Received in revised form 22 September 2014 Accepted 7 October 2014
Hot carrier solar cells is an attractive technology with the potential of reaching high energy conversion efficiencies approaching the thermodynamic limit of infinitely stacked multi-junction solar cells: 65% under one sun and 86% under maximally concentrated. The hot carrier solar cell is conceptually simple consisting of two key components: absorber and energy selective contacts. High efficiencies are achieved by minimising the energy lost to thermalisaton of hot photo-generated carriers while absorbing majority of the solar spectrum. For this to be achieved, energy selective contacts are required to allow the extraction of carriers fast enough at an energy level above the electronic band edge. It is critical for the absorber to be able to maintain a hot carrier population for a sufficiently long time period for the extraction of carriers while they are ‘hot’. Bulk materials with a large gap between acoustic and optical branches in the phonon dispersion are predicted to exhibit slow hot carrier thermalisation rates. Hafnium nitride is such a material with a large gap in its phonon dispersion and is identified as a potential material to be used as a hot carrier absorber. Hafnium nitride has been deposited using reactive sputtering and characterised to investigate material properties and carrier cooling rates. & 2014 Published by Elsevier B.V.
Keywords: Hot carrier solar cell Hafnium nitride Phonons Hot carriers Carrier cooling Transient absorption spectroscopy
1. Introduction The optimal semiconductor bandgap for a solar cell to achieve maximum energy conversion efficiency is characterised by a trade-off between energy lost to the lattice via hot carrier thermalisation and photons with energies below the bandgap not being absorbed [1]. Thermalisation occurs when hot carriers relax to the conduction band edge from elevated energy levels above the conduction band. Hot carriers refer to electrons or holes with energies above the conduction band edge or below the valance band edge. The distribution of these energetic carriers can be described by the Fermi–Dirac distribution at a higher temperature, hence the name hot carriers. Thermalisation of hot carriers in photovoltaic devices makes up a significant portion of energy lost as heat, which cannot be converted to electricity. At the optimal bandgap of 1.3 eV as described by the Shockley– Queisser limiting efficiency of p–n junction solar cells, the energy lost to thermalisation is 30% and below bandgap losses is 25% of the incident illumination [2]. The hot carrier solar cell aims to
n Correspondence to. TETB (building H6), University of New South Wales, Sydney, NSW 2052, Australia. Tel.: þ 61 4 2308 6933. E-mail address: [email protected] (S. Chung).
address these two fundamental loss mechanisms that limit the efficiency of conventional p–n junction solar cells [3–7]. The hot carrier solar cell's objective is to capture the hot carrier population of photo-generated electron hole pairs before thermalisation processes occurs. This also enables a low electronic bandgap material to be used as a photon absorber, thus minimising energy loss due to sub-bandgap photons not absorbed. The hot carriers are separated and extracted to the external circuit by the use of energy selective contacts, where only carriers of a narrow range energies elevated above the absorber's bandgap are collected. It is required to extract the carriers at a narrow range of energies to minimise the entropy generation during carrier collection, which is to prevent the cooling of hot carriers while in contact with cold carriers in the metal contact. The absorber is required to maintain a hot carrier population for a long enough time for the hot carriers to be extracted. The two key requirements of a hot carrier solar cell as shown in Fig. 1 includes an absorber with bandgap, for example, where the carrier population, n(ε), remains at elevated temperatures and energy selective contacts, ESCs where carriers are extracted with a narrow band at a high voltage. The difference in potential between the two electrodes Ve and Vh represents the open circuit voltage of such a device. We investigate the properties and carrier dynamics in hafnium nitride (HfN) in the context of an absorber layer for the Hot Carrier
http://dx.doi.org/10.1016/j.solmat.2014.10.011 0927-0248/& 2014 Published by Elsevier B.V.
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solar cell. While HfN's electronic properties are metallic as shown by the calculated electronic structure in Fig. 2(a) from Saha et al. [9], it has a potential to exhibit slow carrier cooling due to its phononic properties. Energy selective contacts are required to collect carriers at an elevated level above the electronic bandgap, thus it may be possible to design a hot carrier solar cell using an absorber with no electronic bandgap if the energy selective contacts are able to extract carriers at elevated energies and the hot carriers are maintained long enough in the absorber to be extracted. Materials that exhibit long hot carrier lifetimes may also be used in technologies, such as thermoelectric, plasmonic and thermionic emission devices [10–12].
2. Thermalisation of hot carriers and slowed carrier cooling In a semiconductor, electrons are excited to the conduction band from the valence band if the incident photon energy is larger than the semiconductor's bandgap. Elastic carrier–carrier scattering occurs within several femtoseconds, which redistribute the energy of the hot carrier population to a Fermi–Dirac distribution described by a temperature elevated above the lattice temperature. The hot carrier population subsequently thermalises to the band edge, which occurs from around 10 ps onwards in conventional semiconductor materials used in solar cells [13]. The hot carrier solar cells' efficiency is strongly influenced by the carrier thermalisation lifetime. For the hot carrier solar cell to exceed the Shockley–Queisser limit under 1 sun irradiation the thermalisation time is required to be around 1 ns, and under 1000 sun irradiation, it is below the limiting efficiency of triple junction cells [14].
HfN requires a carrier thermalisation time of the order of 1 ns to be suitable to be used as a hot carrier solar cell absorber, and because HfN does not contain an electronic bandgap, it would be required to be operated under highly concentrated irradiation for its efficiency to be comparable with existing multi-junction cells. The mechanism for thermalisation involves interactions between electrons and optical phonons as described by Fröhlich interactions [15]. Energetic carriers emit optical phonons that quickly decay into acoustic phonons. This is the point where the energy is lost and irretrievable to a solar cell because acoustic phonons have a large group velocity and are able to cross interfaces easily. Decay of high energy optical phonons to lower energy acoustic phonons is dominated by three particle processes known as Klemens decay and Ridley decay. These decay mechanisms adhere to conservation of energy and momentum laws. Klemens decay occurs when a phonon in the optical branch decays into two acoustic phonons with half the energy of the original phonon but each with opposite and equal momentum [16]. Ridley decay occurs when an optical phonon decays to an optical phonon of lower energy and an acoustic phonon while conserving momentum [17]. It is proposed that by preventing the decay of optical phonons to acoustic phonons, slowed carrier cooling rates can be achieved [18]. If there exist no pathway for optical phonons to decay into acoustic phonons a hot optical phonon population can be maintained, also known as a phonon bottleneck. In this situation electrons have enough time to reabsorb optical phonons hence maintaining a hot carrier population. A gap between the optical and acoustic branches of the phonon dispersion such that the minimum of the optical branch is at least twice that of the maximum of the acoustic branch can remove the dominate decay pathways, Klemens decay. Hafnium nitride is a material with a wide phonon gap because of the difference in mass between the constituent atoms and the highly symmetric rock-salt crystal structure [9]. Fig. 2(b) shows the large gap in the calculated phonon dispersion of hafnium nitride.
3. Hafnium nitride deposition
Fig. 1. Schematic and energy diagram of a hot carrier solar cell (from Takeda et al. [8]).
Thin films of HfN were deposited on Si(1 0 0) and quartz substrates using an AJA ATC-2200 magnetron sputtering system by DC sputtering of a 4 in. Hf target of 99.9% purity in a mixed argon and nitrogen atmosphere. An argon gas flow rate of 15 sccm mixed with nitrogen gas flow of 1.5 sccm was used. Before the deposition the chamber was pumped down to a base pressure of lower than 2.5 10 6 Torr. The samples were deposited with constant DC power of 50 W and at a pressure of 1.5 mTorr. Deposition occurred with the substrates heated to approximately 450 1C. The films were deposited for 1 hour. The substrates were
Fig. 2. First-principles calculations of (a) electronic structure and (b) phonon dispersionof hafnium nitride. The large gap between the acoustic and optical phonon dispersion curves is evident. Reproduced from Saha et al. [9] with permission.
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chemically cleaned using RCA1, RCA2 solutions and HF dip for silicon substrates, rinsed in DI water and blown dried with N2 just prior to loading into the load lock chamber. The Hf sputter target was sputter cleaned for, at least 10 minutes, to remove surface oxides and other impurities prior to loading the samples from the load lock to the main chamber. Prior to the film deposition, the substrates were thermally degassed at 550 1C at the base pressure of 2.5 10 6 Torr for 1 hour. Thickness of the films was measured to be approximately 45 nm with a Dektak surface profilometer. The composition of the deposited HfN films was measured using X-ray photoelectron spectroscopy, ESCALAB250Xi from Thermo Scientific with an Al Kα X-ray source. The sample was etched for 270 seconds using a 1 keV Argon ion beam to study the bulk of the HfN films. After the 270 s etch XPS estimates the bulk of the film contained 52.3% hafnium, 43.1% nitrogen and 4.6% oxygen. The chemical composition estimates were calculated by scaling the area under the respective peaks by a scaling factor. Fig. 3 shows the XPS spectra of the Hf 4 f, O 1 s and N 1 s peaks of the studied HfN sample. The locations of the Hf 4 f5/2 and 4f7/2 peaks are at 15.1 eV and 16.5 eV, respectively, which agree with work done by Perry et al. [19]. The O 1 s peak is located at 532.1 eV with a broad tail at higher energies. The cause for this broad peak near the O 1 s is unknown; possible explanations may be oxygen bonding with non-Hf impurities in the film. The binding energy of the N 1 s peak is located at 397.5 eV, which is as that for HfN reported in the literature [20]. It is presumed that the nitrogen content of the film is underestimated due to the preferential sputtering of nitrogen over hafnium atomic species. It is possible the oxygen contamination in the film occurred from oxygen impurities in the deposition chamber and oxidation of the sample surface in air. It should, however, be noted that compositional information from XPS is qualitative and method such as Rutherford Backscattering Spectroscopy is required for the accurate determination of composition. The effect of oxygen and other defects in the material is expected to decrease carrier lifetimes by introducing additional phonon modes in the phonon gap allowing decay processes to occur. A PANalytical Empyrean diffractometer with a Cu Kα X-ray source was used for ω-2θ and grazing incident X-ray diffraction (GI-XRD) scans for the analysis of the film structure. HfN films deposited on silicon exhibited a preferred growth orientation in [1 0 0] direction as evident from the ω-2θ XRD measurements, where only the HfN(2 0 0) peak is visible (see Fig. 4). GI-XRD measurements on the samples grown on Si(1 0 0) and quartz was
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done to investigate the bulk of the film and whether they are single phase. GI-XRD scans show HfN(1 1 1), HfN(2 0 0), HfN(2 2 0) and HfN(3 1 1) peaks, suggesting polycrystalline films, as seen in Fig. 5. Epitaxial growth of HfN on silicon is not expected due to a large lattice mismatch between the two materials, about 17%. In comparison to the films deposited on Si(1 0 0), the films on quartz substrates are polycrystalline with less preferential to growth in the [1 0 0] direction as higher order HfN peaks are more pronounced. Fig. 5(a) shows GI-XRD measurements of a typical HfN film deposited on a quartz substrate, with a large substrate peak at 211, HfN(1 1 1), HfN(2 0 0), HfN(2 2 0) and HfN(3 1 1) peaks. While in Fig. 5(b) for HfN film deposited on a Si(1 0 0) substrate, the HfN peaks are similar but there exists a broad peak at 301, which according to Seo et al. [21] is evidence of a nitrogen-rich phase of HfN in contrary to estimations of the film composition from XPS. From the XRD measurements there is no evidence of HfO2 peaks as the presence of oxygen is suggested by XPS. Raman spectra measurements were performed using a Renishaw inVia Raman microscope. Raman modes were excited by the 514 nm line of an Ar þ laser. As shown in Fig. 6, Raman spectroscopy results show agreement between the Raman spectrum and the calculated phonon dispersion of the HfN films. The Raman peak at 520 cm 1 is associated with zone centred
Fig. 4. ω-2θ X-ray diffraction spectrum of a hafnium nitride film deposited on Si (1 0 0) substrates. The signal intensity in the range of 251–651 has been multiplied by 100 times to facilitate comparison with the substrate, Si (4 0 0) peaks. The peak at 39.51 represents the HfN (2 0 0) peak and the small peak at 331 is an artefact of the substrate.
Fig. 3. XPS spectra of (a) Hf 4 f and (b) O 1 s and N 1 s peaks of hafnium nitride films deposited via sputtering.
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Fig. 5. Grazing incident X-ray diffraction spectrum of hafnium nitride on (a) quartz and (b) silicon substrate. The large peak 211 is that of the quartz substrate and the following peaks at 34.111, 39.571, 57.321 and 68.421 corresponds to that of (1 1 1), (2 0 0), (2 2 0) and (3 1 1) peaks of hafnium nitride. Presence of various HfN peaks demonstrates that the HfN film is polycrystalline.
Fig. 6. Raman spectra of deposited hafnium nitride film on (a) silicon substrate and (b) quartz substrate. A represents the acoustic peak and O represents the optical peak.
optical mode and Raman peak at 120–160 cm 1 can be associated with the acoustic peaks of HfN. In Fig. 6(a), for the Raman spectra of HfN deposited on Si(1 0 0) substrates, additional modes are observable, second-order acoustic modes between 230–340 cm 1, optical þ acoustic modes at 440 cm 1 and optical þacoustic modes at 650 cm 1 a, similar to the Raman spectra observed by Stoehr et al. [22] for overstoichiometric HfNx films. Although observations of first-order acoustic peaks in rock-salt structured HfN are not expected through Raman spectroscopy, the acoustic peak may be attributed to non-stoichiometric HfN polycrystalline films leading to point defects breaking the crystal symmetry leading to optical excitations of acoustic modes [22]. The large gap between the acoustic and optical Raman modes is associated with the corresponding gap in the phonon dispersion of hafnium nitride arising from the mass difference between hafnium and nitrogen atomic species. The peaks in the Raman spectrum of HfN deposited on quartz is relatively broad as shown in Fig. 6(b); the Raman spectra of samples deposited on silicon substrates displayed sharper and more defined peaks. This is an indication of the quality of the sample where a broad range of excitations in the phonon modes exist. Dominant Raman peaks associated with HfO2
are found in at 140–200 cm 1 and 500 cm 1, which also coincides with observed HfN Raman peaks [23], thus Raman scattering cannot be used to reliably distinguish the presence of HfO2 from HfN. The transmission and reflection of the HfN thin film deposited on quartz was measured using an integrating sphere with a PerkinElmer LAMBDA1050 UV/Vis/NIR spectrophotometer. From the transmission and reflection optical spectroscopy data, the absorption coefficient α of the HfN film was calculated using the following approximation, 1 T cm 1 α ln d ð1 RÞ where d is the film thickness, T is the transmission and R is the reflection of the thin film as measured by the spectrometer. Fig. 7 shows the absorption coefficient of HfN deposited on quartz. High absorption at long wavelengths is a characteristic of free electron absorption. The minimum absorption coefficient at 440 nm represents the transition point from free electron absorption and reflection to inter-band electron transitions for higher energies. It is at this point where the photo-absorbed electrons are energetic
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y = A1*exp(-x/t1) + y0
0.0035
HfN 400 nm Value
0.0030
5.69058E-4
A1
7.9332E-4
t1
0.0025
ΔOD
y0
470.55
1737.60169
0.0020 0.0015 0.0010 0.0005 0.0000
Fig. 7. Absorption coefficient, α, of hafnium nitride deposited on quartz calculated from transmission and reflection spectroscopy.
Fig. 8. The variation in the optical density spectra obtained from transient absorption spectroscopy at different probe time delays. Measurements of the hafnium nitride deposited on quartz were performed using a pump pulse of 600 mW at 400 nm.
enough to transition from N 2p-states below the Fermi level to Hf 5d-states at the Fermi level in the electronic band structure of HfN. At these higher photon energies above the electronic gap, 2.8 eV at the gamma point as shown in Fig. 2(a) strong absorption occurs.
4. Carrier lifetime measured using ultrafast transient absorption spectroscopy The femtosecond pump-probe experiments were performed on HfN samples deposited on quartz substrates with a transient absorption (TA) spectrometer (FemtoFrame II, IB Photonics). The excitation source was a 400 nm pulse of 600 mW by an OPA laser (TOPAS, Spectra Physics) with 100 fs duration and 1 kHz repetition rate. The probe beam of white light continuum was generated by focusing the 1 kHz Ti:Sapphire-amplified laser (Spitfire, Spectra Physics) into BBO crystal and detected using a polychromator-CCD. Fig. 8 shows the TA spectrum at various time delays, showing the change in optical density, ΔOD, as a function of the probe wavelength. Immediately after the ultra-short laser irradiation, electrons are excited to high excited states. Ultrafast electron– electron scattering occurs extremely within a few hundred femtoseconds. An excited state absorption peak appears at 470 nm, a bleaching band appears between 500–700 nm and broad bleaching at wavelengths greater than 700 nm. These excited absorption and bleaching peaks on these time scales suggest that the HfN sample contains many free electrons for absorption and thus there is effective electron–electron scattering between these electrons. At longer time scales from several femtoseconds up to a couple of nanoseconds a stable linear-like distribution forms in the TA
0
500
1000
1500
2000
Time delay(ps) Fig. 9. Time evolution of the variation of optical density in hafnium nitride and single exponential fitting obtained from transient absorption spectroscopy measurements.
spectrum, from 450 to 750 nm. This linear-like distribution takes several nanoseconds to relax back to the steady-state absorption levels. The linear-like distribution of the TA spectrum may possibly be representative of the tail end of a bleaching peak at a wavelength beyond the range of our data and with this tail the Fermi–Dirac distribution of electrons described by an elevated temperature. Fig. 9 presents the time evolution of the change in optical density at 470 nm with exponential fitting obtaining a decay time constant of 1.7 ns. Exponential fitting of wavelengths around the excited state absorption peak 460–490 nm and the bleaching between 710 and 740 nm yielded time constants from 0.66 ns up to 3 ns. Further analysis of the TA data is required to extract the relevant carrier temperatures and the associated carrier cooling rate. The effect of how the optical density changes in response to the laser pump would need to be modelled with respect to the material's electronic density of states. Carrier cooling rates in HfN are expected to be on similar time scales to that of the exponential fitting for the exponential fitting of these probe wavelengths. It is hypothesised that the long relaxation times are due to the large mass difference between the constituent atoms of HfN leading to a large gap between the optical and acoustic branches in the phonon dispersion. The gap in the phonon dispersion in HfN reduces carrier thermalisation rates by removing phonon decay pathways creating a phonon bottleneck effect allowing electrons to reabsorb optical phonons. Materials that exhibit extremely slow carrier thermalisation rates can be used for the development of the hot carrier solar cell, which can theoretically achieve very high power conversion efficiencies. Present hot carrier solar cell models predicts a device using an absorber material with carrier cooling rates of around 1 ns can yield efficiency greater than 50% under 1000 sun illumination. Under practical considerations the efficiency that can be achieved using HfN will be less than 50% because the model assumes an ideal case where there are no contact losses, perfect light absorption and other possible loss mechanisms. For a device to be made using HfN as an absorber energy selective contacts must be incorporated. Potential avenues for carrier selection are resonant tunnel contacts made with a thin layer between two insulating layers where resonant tunnelling can occur at a specific energy level [24].
5. Conclusion Growth of hafnium nitride thin films on silicon and quartz were done by reactive sputtering. The chemical composition of the HfN films was analysed using XPS. It showed the presence of small amounts of oxygen. For a device to be fabricated, encapsulation of
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HfN with an oxygen barrier may be required to prevent oxidation of the material, and this may also be used as a thin insulating layer for energy selective tunnel contacts. HfN deposited on Si(1 0 0) are polycrystalline and grow preferentially HfN(1 0 0) grains, while HfN deposited on quartz substrates exhibited lower crystallinity quality. Preliminary evidence from ultrafast TA spectroscopy has shown evidence of long carrier lifetimes, on the order of nanoseconds. It is worthwhile to note the quality of the samples studied can be improved, such as by deposition HfN epitaxially on MgO substrates where there is lower lattice mismatch. It is expected that epitaxial HfN will yield better results than presented here. Acknowledgements This work has been supported by the Australian government through the Australian Renewable Energy Agency (ARENA). Responsibility for the views, information or advice expressed herein is not accepted by the Australian government. References [1] W. Shockley, H.J. Queisser, Detailed balance limit of efficiency of p-n junction solar cells, J. Appl. Phys. 32 (3) (1961) 510. http://dx.doi.org/10.1063/1.1736034. [2] L.C. Hirst, N.J. Ekins-daukes, Fundamental losses in solar cells, Prog. Photovoltaics Res. Appl 19 (3) (2011) 286–293. http://dx.doi.org/10.1002/pip. [3] R. Ross, A. Nozik, Efficiency of hot-carrier solar energy converters, J. Appl. Phys. 53 (5) (1982) 3813. [4] P. Würfel, A.S. Brown, T.E. Humphrey, M.A. Green, Particle conservation in the hot-carrier solar cell, Prog. Photovoltaics Res. Appl 13 (4) (2005) 277–285. http://dx.doi.org/10.1002/pip.584. [5] M.A. Green, Third Generation Photovoltaics : Advanced Solar Energy Conversion, Springer-Verlag Berlin and Heidelberg GmbH & Co. KG, 2003. [6] P. Aliberti, Y. Feng, Y. Takeda, S.K. Shrestha, M.A. Green, G. Conibeer, Investigation of theoretical efficiency limit of hot carriers solar cells with a bulk indium nitride absorber, J. Appl. Phys. 108 (9) (2010) 094507. http://dx.doi.org/ 10.1063/1.3494047. [7] Y. Feng, P. Aliberti, B.P. Veettil, R. Patterson, S. Shrestha, M.A. Green, G. Conibeer, Non-ideal energy selective contacts and their effect on the performance of a hot carrier solar cell with an indium nitride absorber, Appl. Phys. Lett. 100 (5) (2012) 053502. http://dx.doi.org/10.1063/1.3680594. [8] Y. Takeda, T. Motohiro, D. König, P. Aliberti, Y. Feng, S. Shrestha, G. Conibeer, Practical factors lowering conversion efficiency of hot carrier solar cells, Appl. Phys. Express 3 (10) (2010) 104301. http://dx.doi.org/10.1143/APEX.3.104301.
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