NAMBE2016 Session MBE-MoM: Oxide Materials (8:30 am-10:15 am)/Solar Energy (10:45 am-12:00 pm)

Transparent conductive oxide β-Ga₂O₃ has attracted extensive attention for next generation of UV opto-electronics and high temperature sensors due to its wide band gap (~4.9eV) [1]. This new semiconductor material has higher breakdown voltage (8MV/cm) and n-type conductivity which make it suitable for potential application as high power electronics [2]. Among various techniques, molecular beam epitaxy technique has been confirmed as a very successful thin film deposition method because of its capability to reduce film impurity levels with improve crystalline quality [3]. Two methods can be used to grow Ga₂O₃ using MBE; one method is to use elemental Ga with RF plasma and the second is the use of a polycrystalline Ga₂O₃ compound source with RF plasma. In this study, we will report the growth and characterization of β-Ga₂O₃ thin film using Ga₂O₃ compound source on c-plane sapphire and investigate the influence of the growth temperature on crystalline quality and surface. The crystalline quality and the film density are determined by high-resolution XRD rocking curve and x-ray reflectivity while chemical composition and optical properties are determined by x-ray photoelectron spectroscopy (XPS) and spectroscopic ellipsometry (SE), respectively. To explore the relation between lattice vibration modes and emission processes in β -Ga₂O₃, polarized Raman spectra will be presented.

The growth temperature was varied from 500 to 850°C to grow β -Ga₂O₃ using compound Ga₂O₃ source with oxygen plasma using a flow rate for oxygen to be 45 sccm and a plasma power of 300W. For growth temperatures above 600°C the Ga₂O₃ layer is single crystal whereas for lower growth temperatures, RHEED observations suggest poly-crystal films. The X-ray diffractometer 2q scan showed the crystalline films to be (`210) oriented monoclinic β-Ga₂O₃ on c-plane sapphire. The thickness of the film was also varied with substrate temperature possibly due to the desorption of Ga₂O molecules. A positive shift was found in the binding energy (BE) for the Ga 2p peaks in XPS measurement which is consistent with a Ga being in a 3+ oxidation state. The electrical properties were measured both in the dark as well as under visible and UV illumination using a semiconductor parameter analyzer (Agilent, 4155C). These experimental results suggest excellent structural quality and packing density of the oxide films and combined with the I-V characteristics under UV light irradiation points to β-Ga₂O₃ thin films as a potential candidate for deep-ultraviolet photodetectors.

Wide bandgap semiconductor materials can enable high -voltage power electronic devices with significantly reduced device dimensions and higher efficiency compared to current technologies. β-Ga₂O₃ is a wide-bandgap semiconductor with a large bandgap (~ 4.5-4.9 eV) and a high breakdown field (~8 MV/cm) [1]. Availability of high crystalline quality β-Ga₂O₃ bulk substrates and recent device demonstrations make this material extremely attractive to realize an economically feasible high power technology [1]. One drawback of β-Ga₂O₃ is its poor thermal conductivity [2]. Furthermore, unlike III-Nitrides, a 2-dimensional electron gas (2DEG) at a β-Ga₂O₃ hetero-I nterface is yet to be demonstrated. In this work, we propose AlN as a n interesting candidate as a gate dielectric for Ga₂O₃ power devices. AlN has a large bandgap (~ 6eV), high thermal conductivity (2.85 Wcm^-1K^-1) and spontaneous/piezoelectric polarization along [002] orientation [3]. These properties can lead to large gate breakdown fields, efficient heat removal, and possible formation of 2DEG at the AlN/ Ga₂O₃ h eterojunction. Since (002) polar plane of AlN shares the 6-fold symmetry of the (-201) plane of β-Ga₂O₃, this crystal orientation can host epitaxial (002) AlN films. In this work, we successfully grew AlN films on (-201) unintentionally doped (UID) bulk β-Ga₂O₃ substrates using MBE (Fig.1a). XRD (Fig.1b), Raman spectroscopy, and RHEED analysis confirm growth of polycrystalline AlN films with a preferred (002) orientation. To study the AlN/Ga₂O₃ interface, we fabricated vertical Schottky barrier diodes (SBD) (Fig.1a). The capacitance-voltage measurements (Fig.1c) suggest the presence of a 2DEG at the AlN/β-Ga₂O₃ interface. This work is the first demonstration of growth of AlN on bulk β-Ga₂O₃ substrates and shows the promise of AlN as gate dielectric for Ga₂O₃ power devices.

This work was supported by NSF DMREF program (Award Number 1534303) and made use of CCMR (NSF MRSEC program (DMR-1120296)) and CNF Facilities (NSF NNCI program (ECCS-1542081).

⁺ Author for correspondence: njt47@cornell.edu [mailto:njt47@cornell.edu] and averma@cornell.edu [mailto:averma@cornell.edu]

[1] M. Higashiwaki et al., Semicond. Sci. and Technol. 31, 034001(2016), [2] Z. Guo et al., Appl. Phys. Lett., 106, 111909 (2015), [3] G. Slack et al., J. Phys. Chem. Solids 48, 641-647 (1987)

We will present on the growth of phase-pure, epitaxial BaSnO₃ films using a hybrid molecular beam epitaxy (MBE) approach with scalable growth rates. In this approach, we use a metal-organic precursor (hexamethylditin) as a tin source, a solid effusion cell for barium, and an rf plasma source for oxygn. BaSnO₃ films were grown on SrTiO₃ (001), LaAlO₃ (001) and LSAT (001) substrates. The substrate temperature and oxygen pressure were kept fixed at 900 C, and 5x10^-6 Torr respectively whereas Ba/Sn beam equivalent pressure (BEP) ratio was varied to optimize cation stoichiometry. The unstrained lattice parameter determined using high-resolution X-ray diffraction, and the Rutherford backscattering spectroscopy (RBS) were used to optimize cation stoichiometry. Lanthanum was used as n-type dopants.

Stoichiometric composition yielded an unstrained lattice parameter value of 4.116±0.001Å, which is identical to that of bulk BaSnO₃. This value was found to increase for Ba-rich films whereas Sn-rich films resulted into secondary phase formation. Time-dependent reflection high-energy electron diffraction (RHEED) intensity oscillations were observed during film growth indicating films grew in a layer-by-layer fashion. Atomic force microscopy confirmed smooth surface morphology for stoichiometric films. Non-stoichiometry films showed surface nano crystallites which correlated with the film stoichiometry. Most remarkably, phase-pure BaSnO₃ could also be grown with the molecular oxygen (i.e. without any rf plasma) suggesting an important role of reactive radical chemistry during film growth. We will discuss these results in the context of highly reactive Sn radicals growth mechanism that assist with the reaction and compound formation.

Finally, we will present a comprehensive electrical characterization of La-doped BaSnO₃ and will discuss how electrical transport is influenced by the presence of structural defects such as dislocations, non-stoichiometry, and dopant concentration. We will also present different scattering mechanisms in La-doped BaSnO₃ that limits the room temperature electron mobility. We will present pathways to suppress these scattering rates - a step closer towards defect-managed high mobility oxide thin films and heterostructures.

As Si solar technology approaches ~26% efficiency, integrating III-V devices onto Si in a multijunction architecture has emerged as a promising pathway towards surpassing Si’s inherent single-junction (1J) efficiency limit, while reducing the cost of III-V devices. 1.7 eV GaAs_yP_1-y (hereafter GaAsP) offers both an optimal bandgap (E_g) for a tandem device on Si [1], as well as a convenient lattice constant grading route by means of a GaP nucleation layer on Si, followed by a GaAs_yP_1-y graded buffer. The previous GaAsP/GaP/Si 1J solar cell efficiency record was only 9.8% [2], largely stunted by poor short-circuit current densities (J_SC) and open-circuit voltage (V_OC). A major factor limiting the efficiency is high threading dislocation densities (TDD) >10⁷ cm^-2 in these materials, caused by the large ~3% lattice mismatch between GaAsP and Si. By optimizing the MBE growth of our GaAs_yP_1-y graded buffers and implementing an improved device design, we have surpassed the previous GaAsP/GaP/Si 1J efficiency record, routinely achieving upwards of 11.5-12.0% efficiencies without an anti-reflection coating (ARC, Fig. 1).

Our devices were grown in a solid source MBE chamber on GaP/Si templates that have a thin layer of pseudomorphic GaP on a bulk Si (001) wafer. On these templates, a relaxed 500 nm GaP buffer was grown at 505°C, followed by a GaAs_yP_1-y graded buffer, and a GaAsP solar cell at 575°C. In order to minimize TDD, we investigated various graded buffer growth temperatures (505-630°C) and grading rates (0.38-0.80 %misfit/μm) while keeping the GaP buffer and solar cell growth conditions constant. As a result of our investigation, we achieved the lowest reported TDD in GaAsP/GaP/Si solar cells of 4.0-4.6×10⁶ cm^-2, which has enabled improved V_OC’s of 1.13-1.15 V. Additionally, we greatly enhanced the current collection in our devices by implementing a more transparent, higher-bandgap InAlP window layer and thinner emitter compared to past years, which has enabled record uncoated J_SC values greater than 13 mA/cm² (Fig. 1 & 2). We have recently begun work on design and integration of an ARC on our cells, and recently achieved a new NREL-certified record efficiency of 14.0%. Overall, we have significantly advanced the current state of GaAsP/GaP/Si 1J solar cells through dislocation and design engineering and established a realistic path towards 2J cells with >30% efficiency.

⁺ Author for correspondence: michelle.vaisman@yale.edu

[1] J. F. Geisz, et al. Semicond. Sci. Tech. 17, 769 (2002).

[2] J. F. Geisz, et al. IEEE Photovolt. Spec. Conf. 772 (2006).

This work explores quantum dot-well (QD+QW) hybrid structures in which QDs couple to an adjacent QW through a thin spacer layer. We show that this kind of heterostuctures can be used in the active region in a solar cell to increase infrared (IR) light absorption. Several (QD+QW) hybrid structures have been fabricated by a molecular beam epitaxy (MBE) and have been studied for their morphology and optical properties, including the (InAs/GaAs QDs + InGaAs/GaAs QW), (GaSb/GaAs QDs + InGaAs/GaAs QW), (GaSb/InAlAs QDs + InGaAs/InAlAs QW), (GaSb/AlGaAs QDs + GaAlAs/GaAs QW) structures. Compared to their reference QD structure, we observe that all the hybrid structures have the PL emission shifting to a longer wavelength. The hybrid structure exhibits strong absorption/emission efficiency due to excess carriers generated in the QW and then tunneling into the QDs. These hybrid structures hence offer an alternative way harvest longer wavelength photons in solar cells. [1-2] We have investigated the performance of a solar cell device by incorporating a (GaSb/GaAs QD + InGaAs/GaAs QW) hybrid structure into i-region of a p-i-n cell. Various device parameters are measured from QW, QD, (QW+QD), and control (without nanostructures) solar cells. We see that the device performance degrades with the insertion of nanostructures due to strain induced effects and non-radiative recombination. However, no further degradation in VOC is observed in (QW+QD) solar cells in comparison with the device containing only QDs. Moreover the IR photocurrent from the (QW+QD) solar cell surpasses the photocurrents in reference QD and QW solar cells, indicating that this combination of QDs and a QW improves IR photon absorption. [3]

+ Author for correspondence: bliang@@cnsi.ucla.edu

[1] R.B. Laghumavarapu, M. El-Emawy, N. Nuntawong, A. Moscho, L.F. Lester, and D.L. Huffaker, Appl. Phys. Lett. 91, 243115 (2007).

[2] A. Martí, N. López, E. Antolín, E. Cánovas, A. Luque, Appl. Phys. Lett. 90, 233510 (2007).

[3] R. B. Laghumavarapu, B.L. Liang, Z. Bittner, T. S. Navruz, S. Hubbard, D. L. Huffaker, Solar Energy Materials and Solar Cells, 114, 165-171 (2013).

Multijunction solar cells have been studied to realize ultra high efficiency solar cells. 4- and 5-junction solar cells realized by a directly bonding technique have the highest reported efficiencies of 44.7 and 38.8% under concentrator and AM1.5 conditions, respectively. Above multijunction structures are grown by MOCVD because materials are needed that include phosphorus such as InGaP and InGaAsP. However, there have been few reports on InGaP and multijunction solar cells fabricated using solid-source molecular beam epitaxy (SS-MBE) because the phosphorus based materials are very difficult to grow. In our earlier work, we reported InGaP, InGaP/GaAs tandem, InGaP/(In)AlGaAs/GaAs 3-junction solar cells grown using SS-MBE [1, 2]. We have also proposed a new semiconductor bonding technology for mechanically stacked multijunction solar cells by using conductive nanoparticle alignments called smart stack technology [3]. This technique is very attractive to interconnect different kinds of solar cells. In this paper, we report InGaAsP (1.65eV) second cells as an Al-free material grown using SS-MBE for the first time . Moreover, we report InGaP/InGaAsP/GaAs 3-junction top sells for smart stack multijunction solar cells.

In_0.27Ga_0.72As_0.44P_0.56 (1.65eV) solar cell structures were grown at 450, 430, and 410 °C, respectively on GaAs substrates. A back surface field (BSF) layer and an antireflection coating (ARC) were not employed. The InGaAsP solar cell grown at 430 °C had the highest efficiency of 9.74% without ARC. This cell efficiency is better than that of our previous (In)AlGaAs second cell [2]. We also fabricated InGaP/InGaAsP/GaAs 3-J top cells which have better characteristics than InGaP/InAlGaAs/GaAs top cells [2]. The small short circuit current density (J_sc) is due to the thin absorption layer thickness and the absence of ARC. These results indicate that the SS-MBE grown InGaAsP second cell represent a promising pathway for the future development of high efficiency smart stack multijunction solar cells.

[1] T. Sugaya et al., Jpn. J. Appl. Phys., 53, 05FV06 (2014).

[2] T. Sugaya et al., Jpn. J. Appl. Phys., 54, 08KE02 (2015).

[3] H. Mizuno et al., Appl. Phys. Lett., 101, 191111 (2012).

GaP as electron-selective contact in silicon solar cell has been theoretically shown to enable a boost in conversion efficiency up to 26.7% [1]. However, this requires finding a solution to remediate to the Si bulk lifetime degradation from ms to µs level recently reported during MBE epitaxial growth (Fig.1a) [2]. Here, we present two practical methods to effectively mitigate Si bulk lifetime degradation and apply these to demonstrate the first silicon heterojunction solar cells having a completely amorphous-silicon-free structure, i.e. a molybdenum oxide hole (MoO_x) contact [3] and a GaP electron selective contact (Fig.1b).

Our approach is to concentrate the impurities to a sacrificial region in the Si that is etched away after GaP growth. The first uses a silicon nitride (a-SiN_x:H) coating on the back of Si substrate that acts as a diffusion barrier and enables gettering at the Si/SiN_x interface. This led to 1.5ms Si bulk lifetime after thermal treatment in the MBE chamber and etching the SiN_x (Fig.1a). The second is the formation of a P-rich regions before GaP growth by POCl₃ diffusion; this region acts as a gettering sink for impurities and after etching led to bulk Si lifetime of more than 400µs for the annealed sample (Fig.1a). We used both approaches during epitaxial growth of (n) GaP:Si, and after appropriate etching, prepared full solar cell with a MoO_x hole contact (Table 1). These are a first demonstration and need further optimization, particularly regarding recombination at the GaP/Si interface.

[1] S. Limpert, et al., IEEE 40^th PVSC, 0836-0840 (2014).

[2] C. Zhang, et al., Abstracts of 31st NAMBE conf., TU 13, 42, Cancun, Mexico (2015).