GaAs/AlGaAs Nanowire Array Solar Cell Grown on Si with Ultrahigh Power-per-Weight Ratio

Here we demonstrate a more effective use of III–V photoconversion material to achieve an ultrahigh power-per-weight ratio from a solar cell utilizing an axial p-i-n junction GaAs/AlGaAs nanowire (NW) array grown by molecular beam epitaxy on a Si substrate. By analyzing single NW multicontact devices, we first show that an n-GaAs shell is self-formed radially outside the axial p- and i-core of the GaAs NW during n-core growth, which significantly deteriorates the rectification property of the NWs in the axial direction. When employing a selective-area ex situ etching process for the n-GaAs shell, a clear rectification of the axial NW p-i-n junction with a high on/off ratio was revealed. Such a controlled etching process of the self-formed n-GaAs shell was further introduced to fabricate axial p-i-n junction GaAs NW array solar cells. Employing this method, a GaAs NW array solar cell with only ∼1.3% areal coverage of the NWs shows a photoconversion efficiency of ∼7.7% under 1 Sun intensity (AM 1.5G), which is the highest achieved efficiency from any single junction GaAs NW solar cell grown on a Si substrate so far. This corresponds to a power-per-weight ratio of the active III–V photoconversion material as high as 560 W/g, showing great promise for high-efficiency and low-cost III–V NW solar cells and III–V NW/Si tandem solar cells.publishedVersio

In Situ Heat-Induced Replacement of GaAs Nanowires by Au.

Here we report on the heat-induced solid-state replacement of GaAs by Au in nanowires. Such replacement of semiconductor nanowires by metals is envisioned as a method to achieve well-defined junctions within nanowires. To better understand the mechanisms and dynamics that govern the replacement reaction, we performed in situ heating studies using high-resolution scanning transmission electron microscopy. The dynamic evolution of the phase boundary was investigated, as well as the crystal structure and orientation of the different phases at reaction temperatures. In general, the replacement proceeds one GaAs(111) bilayer at a time, and no fixed epitaxial relation could be found between the two phases. The relative orientation of the phases affects the replacement dynamics and can induce growth twins in the Au nanowire phase. In the case of a limited Au supply, the metal phase can also become liquid.The Research Council of Norway is acknowledged for the support to the Norwegian Micro- and Nano-Fabrication Facility, NorFab (197411/V30), the FRINATEK program (214235), and the NORTEM project (197405). G.D. and C.D. acknowledge funding from ERC under Grant 259619 PHOTO EM. C.D. acknowledges financial support from the EU under Grant 312483 ESTEEM2

Critical thickness of MBE-grown Ga1-xInxSb (x < 0.2) on GaSb

Several Ga1−xInxSb layers, capped with 1 μm of GaSb, were grown on GaSb(0 0 1) substrates by molecular beam epitaxy in a Varian Gen II Modular system using either the conventional sample growth position with substrate rotation, or a tilted sample position with no substrate rotation. The GaInSb layers were examined by X-ray diffraction (XRD) using both symmetrical and asymmetrical reflections. The “tilted sample method” gave a variation of ±25% in thickness of the Ga1−xInxSb layers, while the indium (In) content varied by ±10% around the nominal value. The disappearance of thickness fringes in 004 XRD scans was used to determine the onset of relaxation, as determining the in-plane lattice constant for tilted samples was found to be difficult. Determining residual strain in samples grown by the tilted method was likewise found to be very difficult. The critical thickness for several In mole fractions between 5% and 19% was determined and was found to be from 2.2 to 2.7 times higher than predicted by Matthews and Blakeslee (1974) [J. Crystal Growth 27 (1974) 118] but lower than that predicted by People and Bean (1985) [Appl. Phys. Lett. 47 (1985) 322]

Aluminum-based contacts for use in GaSb-based diode lasers

Aluminum-based contacts could be a good alternative to conventional gold-based contacts for a number of GaSb-based devices. In this study, the use of some Al-based contacts in GaSb-based diode lasers was investigated via the measurement of specific contact resistivity and laser output characteristics. The Al-based contacts to p-type GaSb(001) exhibited lower specific contact resistivities than the conventional Au-based contacts, whereas the opposite was the case for contacts to n-type GaSb(001). The good performance of GaSb-based laser diodes using Al-based contacts shows the applicability of this type of contact in GaSb-based devices. The contact between Al only and p-type GaSb(001), however, could suffer from a reliability problem when used in diode lasers, due to interdiffusion, in which case a diffusion barrier should be included

Plasma-assisted oxide removal from p-type GaSb for low resistivity ohmic contacts

The effect of several plasma-assisted oxide removal techniques prior to metallization of p-type GaSb was investigated. Compared to conventional chemical methods, the plasma-assisted oxide removal resulted in significant improvement of the specific contact resistivities, obtained from transfer length method measurements. Very low specific contact resistivities of less than 5108 X cm2 were observed after surface pretreatment by H2/Ar sputter etching and low-ion-energy argon irradiation. By eliminating sample exposure to air, in situ Ar irradiation becomes a promising technique for high performance GaSb-based semiconductor diode lasers

Epitaxially grown III-arsenide-antimonide nanowires for optoelectronic applications

Epitaxially grown ternary III-arsenide-antimonide (III-As–Sb) nanowires (NWs) are increasingly attracting attention due to their feasibility as a platform for the integration of largely lattice-mismatched antimonide-based heterostructures while preserving the high crystal quality. This and the inherent bandgap tuning flexibility of III-As–Sb in the near- and mid-infrared wavelength regions are important and auspicious premises for a variety of optoelectronic applications. In this review, we summarize the current understanding of the nucleation, morphology-change and crystal phase evolution of GaAsSb and InAsSb NWs and their characterization, especially in relation to Sb incorporation during growth. By linking these findings to the optical properties in such ternary NWs and their heterostructures, a brief account of the ongoing development of III-As–Sb NW-based photodetectors and light emitters is also given

GaAs/AlGaAs Nanowire Array Solar Cell Grown on Si with Ultrahigh Power-per-Weight Ratio

Here we demonstrate a more effective use of III–V photoconversion material to achieve an ultrahigh power-per-weight ratio from a solar cell utilizing an axial p-i-n junction GaAs/AlGaAs nanowire (NW) array grown by molecular beam epitaxy on a Si substrate. By analyzing single NW multicontact devices, we first show that an n-GaAs shell is self-formed radially outside the axial p- and i-core of the GaAs NW during n-core growth, which significantly deteriorates the rectification property of the NWs in the axial direction. When employing a selective-area ex situ etching process for the n-GaAs shell, a clear rectification of the axial NW p-i-n junction with a high on/off ratio was revealed. Such a controlled etching process of the self-formed n-GaAs shell was further introduced to fabricate axial p-i-n junction GaAs NW array solar cells. Employing this method, a GaAs NW array solar cell with only ∼1.3% areal coverage of the NWs shows a photoconversion efficiency of ∼7.7% under 1 Sun intensity (AM 1.5G), which is the highest achieved efficiency from any single junction GaAs NW solar cell grown on a Si substrate so far. This corresponds to a power-per-weight ratio of the active III–V photoconversion material as high as 560 W/g, showing great promise for high-efficiency and low-cost III–V NW solar cells and III–V NW/Si tandem solar cells

Dopant incorporation in Al0.9Ga0.1As0.06Sb0.94 grown by molecular beam epitaxy

Incorporation of beryllium (Be) and tellurium (Te) dopants in epitaxially grown Al0.9Ga0.1As0.06Sb0.94 layers was investigated. Carrier concentrations and mobilities of the doped layers were obtained from room temperature Hall effect measurements, and dopant densities from secondary ion mass spectrometry depth profiling. An undoped Al0.3Ga0.7As cap layer and side wall passivation were used to reduce oxidation and improve accuracy in Hall effect measurements. The measurements on Be-doped samples revealed high doping efficiency and the carrier concentration varied linearly with dopant density up to the highest Be dopant density of 2.9 × 1019 cm−3, whereas for Te doped samples the doping efficiency was in general low and the carrier concentration saturated for Te-dopant densities above 8.0 × 1018 cm−3. The low doping efficiency in Te-doped Al0.9Ga0.1As0.06Sb0.94 layer was studied by deep-level transient spectroscopy, revealing existence of deep trap levels and related DX-centers which explains the low doping efficiency

GaAs/AlGaAs Nanowire Array Solar Cell Grown on Si with Ultrahigh Power-per-Weight Ratio

Here we demonstrate a more effective use of III–V photoconversion material to achieve an ultrahigh power-per-weight ratio from a solar cell utilizing an axial p-i-n junction GaAs/AlGaAs nanowire (NW) array grown by molecular beam epitaxy on a Si substrate. By analyzing single NW multicontact devices, we first show that an n-GaAs shell is self-formed radially outside the axial p- and i-core of the GaAs NW during n-core growth, which significantly deteriorates the rectification property of the NWs in the axial direction. When employing a selective-area ex situ etching process for the n-GaAs shell, a clear rectification of the axial NW p-i-n junction with a high on/off ratio was revealed. Such a controlled etching process of the self-formed n-GaAs shell was further introduced to fabricate axial p-i-n junction GaAs NW array solar cells. Employing this method, a GaAs NW array solar cell with only ∼1.3% areal coverage of the NWs shows a photoconversion efficiency of ∼7.7% under 1 Sun intensity (AM 1.5G), which is the highest achieved efficiency from any single junction GaAs NW solar cell grown on a Si substrate so far. This corresponds to a power-per-weight ratio of the active III–V photoconversion material as high as 560 W/g, showing great promise for high-efficiency and low-cost III–V NW solar cells and III–V NW/Si tandem solar cells