For III-V multi-junction solar cells (MJSC), 1.9 eV Ga_0.51­In_0.49P (hereafter GaInP) is a primary material for the high-bandgap top cell [1]. In recent years, GaInP solar cells grown by molecular beam epitaxy (MBE) have achieved high efficiencies after post-growth rapid thermal annealing (RTA) [2] similar to cells grown by metalorganic chemical vapor deposition (MOCVD). Another avenue to improve MBE-grown GaInP cells is the use of group VI dopants (Se, Te) instead of group IV dopants such as Si. For example, MOCVD-grown AlGaAs and AlGaInP cells reported superior internal quantum efficiency (IQE) by changing the dopant from Si to Se [3], [4], an improvement that correlates with the shallower defect energy levels of Se [5], [6]. MBE-grown phosphide cells may also be improved by the use of a group VI dopant, but such studies have not been reported. In this work, we show improved IQE of GaInP solar cells with Te doping of the n-type emitter over Si, which was modeled to show improved carrier lifetime in n-GaInP:Te.

A well reported issue for MOCVD GaInP:Te is undesirable Te surface segregation [7], so we first optimized MBE growth conditions to minimize segregation. GaInP samples were grown at different target concentrations of [Te] = 5.7*10¹⁷ to 1.7*10¹⁸ cm^-3, and doping profiles from secondary ion mass spectrometry (SIMS) indeed showed high Te surface segregation at the nominal substrate growth temperature T_sub = 460 °C. For example, the doping profile for target [Te] = 1.7*10¹⁸ cm^-3ranged by orders of magnitude (5*10¹⁶ cm^-3 at the start of growth to 7*10¹⁹ cm^-3 at the surface). Next, reducing T_sub from 460 °C to 420 °C greatly suppressed surface segregation for all samples. Abrupt doping profiles were obtained for target [Te] = 5.7*10¹⁷ cm^-3 solely by this reduced T_sub, while [Te] = 1.7*10¹⁸ cm^-3 also required a Te “pre-dose” or deposition prior to GaInP growth to achieve an abrupt doping profile. The cause of this difference in pre-dose requirement as a function of doping will be the subject of future work.

Lastly, n-on-p front junction GaInP cells were grown with both GaInP:Te and GaInP:Si n-type emitters (n > 1*10¹⁸ cm^-3). For as-grown cells, the IQE of the GaInP:Te cell was significantly higher than that of GaInP:Si, with the short-circuit current density derived from IQE increasing from 13.2 mA/cm² to 14.1 mA/cm². Modeling of the IQE indicates a ~4x higher carrier lifetime of GaInP:Te than GaInP:Si. This work shows the promise of GaInP:Te for improved optical material quality, and future work will explore GaInP:Te for rear-junction cells with n = 1–5*10¹⁷ cm^-3, as well as the effect of RTA on GaInP:Te.

Growth of metamorphic (MM) In_xGa_1-xAs enables optoelectronic devices, such as near-infrared photovoltaics, detectors, and lasers with bandgap energies (E_g) ranging from 0.8-1.3 eV, to be integrated on GaAs substrates. Most previously reported MM In_xGa_1-xAs structures with x=0.3 (E_g=1.0 eV) have used metal-organic chemical vapor deposition (MOCVD) at substrate temperatures (T_sub) over 700°C to facilitate high dislocation glide velocity and thus low threading dislocation density (TDD)¹, while thermodynamically suppressing phase separation. MBE growth of MM In_xGa_1-xAs cannot be done at such high T_sub due to excessive In desorption. Thus, the MBE growth window must be chosen carefully to balance the need for adequate dislocation glide velocity while kinetically suppressing phase separation. In this work, we describe a two-step method for fully relaxed In_0.3Ga_0.7As with a TDD = ~7x10⁵ cm^-2, comparable to the lowest reported values for this material. On this platform we fabricated the first MM 1.0 eV In_0.3Ga_0.7As solar cells grown via MBE to our knowledge.

In_xGa_1-xAs (x=0-0.3) buffers showed strong phase separation in electron channeling contrast imaging (ECCI) when all growth was carried out at T_sub=500°C. Therefore, we adopted a two-step approach where In_xGa_1-xAs (x=0-0.18) was grown at T_sub=500°C to facilitate dislocation glide², followed by 460°C for x=0.18-0.3 to prevent PS³; an In_0.35Ga_0.65As overshoot layer was also included to facilitate strain relaxation. ECCI yielded a TDD < 1x10⁶ cm^-3 without PS, while reciprocal space mapping showed 97.9% relaxation in the In_0.3Ga_0.7As cap.

Using our two-step In_xGa_1-xAs graded buffer as a virtual substrate, a 1 eV InGaAs solar cell was grown and devices were fabricated both as-grown (AG) and with rapid thermal annealing (RTA)⁴. Internal quantum efficiency (IQE) measurements showed improved minority electron diffusion length in the base of the RTA cell, resulting in a 1.15x increase in short-circuit current density (J_SC). Lighted current-voltage (LIV) curves showed a strong improvement in the open-circuit voltage (V_OC) after RTA, with a bandgap-V_OC offset (W_OC) of 0.46 V, ~0.1 V higher than the best cells in the literature grown by MOCVD at NREL⁵. The fill factor (FF) and cell efficiency (η) of our RTA’d cells were hampered by high contact resistance, which will be addressed in future runs. Nonetheless, the J_SC (29.96 mA/cm²) and V_OC (0.538 V) values of our RTA’d cells compare favorably with the best lattice-matched 1.0 eV solar cells (e.q. InGaAsN on GaAs, InGaAsP on InP) grown by MBE⁶, presenting a promising path for MBE-grown MM 1.0 eV In_0.3Ga_0.7As solar cells.

Spin-polarized electron beams are a crucial tool in particle physics experiments and specialized characterization techniques. Their production involves exciting a III-V semiconductor (the photocathode) with near-bandgap circularly polarized light. Spin-polarized photoelectrons are produced through optical selection rules. These photoelectrons are then extracted after functionalizing the surface to create a negative electron affinity condition. Increasing the electron spin polarization (ESP) and the quantum efficiency (QE) of the photocathodes enhances overall experimental efficiency, extends the photocathode lifetime, and enables the production of high bunch charge beams. The current state-of-the-art spin-polarized photocathodes consist of GaAs/GaAs_0.67P_0.33 strained superlattice quantum wells grown on metamorphic GaAs_0.67P_0.33 virtual substrates on GaAs(001), which consistently produce QE over 1% and ESP around 85%.

While the GaAs/GaAsP system is mature, it still faces many limitations. Growth of the relaxed virtual substrate results in a higher density of threading dislocations than there would be in a fully pseudomorphic system, thereby limiting QE, ESP, and photocathode lifetime. In addition, since the As/P ratio in the virtual substrate controls the lattice constants, band offsets, and band gaps, it is difficult--if not impossible--to design a stack that optimizes each of these parameters simultaneously.

Here we demonstrate fully pseudomorphic spin-polarized photocathodes based on the InAlGaAs/AlGaAs system grown by molecular beam epitaxy. By switching to this quaternary well/ternary barrier system, we may more independently optimize the strain, superlattice bandgap, and band offsets. Furthermore, control of Group III alloy composition should be more precise, reproducible, and uniform than control of Group V alloy composition. The fully pseudomorphic stack simplifies growth and reduces the dislocation density in the active region. Unfortunately, the transition from a binary/ternary system to a quaternary/ternary system will increase the random alloy disorder in the photocathode. This random alloy disorder can then cause significant reductions in ESP. To solve this, we investigate using a digital alloy for the well and/or barrier in the superlattice. Our first test structure using a random alloy photocathode design yielded polarization over 80%, but a QE of only about 0.3%. We will report on further optimization of the growth conditions of both random and digital alloy photocathodes. In addition, we will discuss the changes to the QE and ESP of the photocathodes between the two alloy schemes.

Heterostructures of a 2-dimensional electron gas (2DEG) semiconductor and a superconductor are prime candidates for various applications including quantum computing and topological superconducting circuits [1,2]. The 2DEG layer needs to be in close proximity to the superconductor, within the structure, and to form an Ohmic contact. Heterostructure of containing a quantum well (QW) of InAs (InGaAs/InAs/InGaAs) that have a narrow bandgap with the Fermi level close to the conduction band as a 2DEG semiconductor and thin epitaxial Al layer grown on it as a superconductor. This heterostructure makes a prime candidate due to the high mobility in the 2DEG and the compatibility between the QW and the epitaxial Al.

In this work, we present our study of the growth of these heterostructures (InAs QW/Al) grown by molecular beam epitaxy [3]. We show the effect of modification of the growth conditions on the heterostructures and analyze the disorder in the samples using different techniques. Various impurities including background impurity, surface impurities, alloy scattering and surface roughness can be moderated as a function of the growth conditions. We find samples that have high indium content in their graded buffer layer show a decrease in surface roughness with growth temperature. In addition, anisotropy along rapid diffusion directions during the growth results in anisotropy in the resulting carrier mobilities parallel with these directions. We also analyze the typical cross hatch pattern of the surface on the InAs heterostructures as a function of growth parameters and relate the strain state anisotropy (or lack thereof) to the observed electrical characteristics. Lastly, we show that growth of a thin capping layer of 1 - 6 nm of In_0.81Al_0.19As modifies the surface scattering on the quantum well, the implications of which will be discussed.

[1] H. Kroemer, Physica E 20, 196 (2004)

[2] J. A. del Alamo, Nature 479, 317 (2011)

[3] W.M. Strickland et al. Appl. Phys. Lett. 121, 092104 (2022)