Selective Area Growth of GaAs Nanowires and Microplatelet Arrays on Silicon by Hydride Vapor-Phase Epitaxy

In this work, we demonstrate the growth of vertically oriented GaAs nanowires (NWs) and microplatelets directly on a patterned SiO2/Si(111) substrate by hydride vapor-phase epitaxy (HVPE). Direct condensation of GaAs on Si was achieved through a critical surface preparation under an As-controlled atmosphere. GaAs NWs were grown along the ⟨111⟩B direction with a hexagonal cross section when the hole opening diameter (D) in the SiO2 mask was below 350 nm. Larger apertures (D ≥ 500 nm) resulted in uniform microplatelets. This study highlights the capability of HVPE for selective area growth of GaAs directly on Si and thus the potential of HVPE as a generic heterointegration process for III-V semiconductors on silicon.</p

Thermal Conductivity of GaAs Nanowire Arrays Measured by the 3&omega; Method

Vertical nanowire (NW) arrays are the basis for a variety of nanoscale devices. Understanding heat transport in these devices is an important concern, especially for prospective thermoelectric applications. To facilitate thermal conductivity measurements on as-grown NW arrays, a common NW-composite device architecture was adapted for use with the 3&omega; method. We describe the application of this technique to obtain thermal conductivity measurements on two GaAs NW arrays featuring ~130 nm diameter NWs with a twinning superlattice (TSL) and a polytypic (zincblende/wurtzite) crystal structure, respectively. Our results indicate NW thermal conductivities of 5.2 &plusmn; 1.0 W/m-K and 8.4 &plusmn; 1.6 W/m-K in the two samples, respectively, showing a significant reduction in the former, which is the first such measurements on TSL NWs. Nearly an order of magnitude difference from the bulk thermal conductivity (~50 W/m-K) is observed for the TSL NW sample, one of the lowest values measured to date for GaAs NWs

Four-point characterization using capacitive and ohmic contacts

A four-point characterization method is developed for semiconductor samples that have either capacitive or ohmic contacts. When capacitive contacts are used, capacitive current- and voltage-dividers result in a capacitive scaling factor which is not present in four-point measurements with only ohmic contacts. Both lock-in amplifier and pre-amplifier are used to measure low-noise response over a wide frequency range from 1 Hz -- 100 kHz. From a circuit equivalent of the complete measurement system after carefully being modeled, both the measurement frequency band and capacitive scaling factor can be determined for various four-point characterization configurations. This technique is first demonstrated with a discrete element four-point test device and then with a capacitively and ohmically contacted Hall bar sample using lock-in measurement techniques. In all cases, data fit well to a circuit simulation of the entire measurement system over the whole frequency range of interest, and best results are achieved with large area capacitive contacts and a high input-impedance preamplifier stage. Results of samples (substrates grown by Max Bichler Dieter Schuh, and Frank Fischer of the WSI) measured in the QHE regime in magnetic fields up to 15 T at temperatures down to 1.5 K will also be shown

Effect of the plasma etching on InAsP/InP quantum well structures measured through low temperature micro-photoluminescence intensity scanning

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Low temperature micro-photoluminescence spectroscopy of microstructures with InAsP/InP strained quantum wells

International audienceRidge microstructures were prepared by reactive ion etching (RIE) of a series of stacked InAs x P 1−x quantum wells (QWs) with step graded compositions grown on InP by molecular beam epitaxy. These microstructures were characterized by low temperature micro-photoluminescence. The photoluminescence (PL) emission associated with each of the QWs was clearly identified and a model for their line shape was implemented. PL line-scans were measured across etched ridge stripes of various widths in an optical cryostat, with a spatial resolution of 1 µm. The model for the PL spectra allowed accurate extraction of the local PL integrated intensities, spectral positions and line widths. Two different RIE processes, using CH 4 /H 2 and CH 4 /Cl 2 , were investigated. The PL line-scans showed strong variations of the integrated PL intensities across the etched stripes. The PL intensities for all QWs increased gradually from the edge to the center of the ridge microstructures, over a length scale of 10 to 20 µm. On the other hand, the spectral peak position of the PL lines remained constant (0.2 to 0.4 meV, depending on which QW was considered) across the microstructures. These observations are discussed in terms of the mechanical stress induced by the RIE processes, the relaxation of the biaxial built-in compressive stress in the InAsP QWs (induced by the free surfaces at the vertical etched sidewalls), and also by the non-radiative recombination at these sidewalls. Altogether, this study illustrates the contribution that specially designed test structures, coupled with advanced spectroscopic characterization, can provide to the development of semiconductor photonic devices (e.g. lasers or waveguides) involving RIE processing

Low-temperature spatially-resolved luminescence spectroscopy of microstructures with strained III-V quantum wells

International audienceThe continuous development of advanced photonic devices based on 3-dimensional structuring of active materials calls for more efforts on characterization techniques. This statement applies, in particular, to processes, such as dry etching applied to III-V semiconductors. Dry (or plasma-based) etching is frequently used within nanofabrication platforms to realize semiconductor lasers, ridge waveguides, photonic integrated circuits, etc. Luminescence techniques (photo-luminescence or cathodo-luminescence) can analyze the material’s properties after dry etching, especially for direct band-gap semiconductors. The presence of non-radiative defects, changes in the local stress/strain, etc., can be probed in detail owing to the spatial resolution of this class of techniques, and their quantification can also be addressed. Our goal in this work is to highlight a robust methodology involving the design of specific test materials, including a series of different quantum wells (QWs) located at well-defined depths below the surface. The spectroscopic signature of these QWs provides valuable information, which can be used to assess the changes occurring to the III-V semiconductor material due to dry etching.The test structures have been designed on (100)-oriented InP substrates, with alternating InAsxP1-x QWs and InP barriers. Sequences of typically 8 QWs with graded As composition were grown, with a fixed thickness (7 to 8 nm), separated by 100 nm InP barriers. The shallowest QW is located 300 nm below the sample surface. By grading the As composition in the QWs (with x typically between 0.35 and 0.5), the luminescence signal for each QW could be unambiguously identified with sharp lines when measured at very low temperatures. These structures were grown by gas-source molecular beam epitaxy. Figure 1 illustrates such a structure.In a second step, a SiNx film was deposited by plasma-enhanced chemical vapor deposition and patterned using standard optical lithography. Elongated stripes were thus defined, whose width varies between 1 and 50 µm, and length is a few mm. Finally, plasma etching was used to fabricate stripes within the QW structures, using this SiNx film as a hard mask. Based on Cl2/CH4/Ar and H2/CH4/Ar, different gas mixtures were employed to perform this etching.We have characterized these etched stripes by micro-PL at 10 K in a specially designed optical cryostat, allowing a spatial resolution of approximately 1 µm and a step-size of 5 µm for mapping the luminescence signal. A 1064 nm laser source was chosen for the excitation of the PL signal in our samples. The choice of this wavelength allows selective excitation of the QWs, avoiding excitation of the InP barrier material. As shown in figure 2, the spectrum displays well-identified lines, which can be attributed clearly to the different QWs in the sample. The very sharp lines (full width at half maximum of the order of 4 meV) attest to the very high sample quality. A fitting procedure was implemented to determine the spectral characteristics of each transition, as illustrated by the red line (“Model”) in fig. 2-a. We have established ([1]) that the observed PL lines are associated with single optical transitions in each QW between the electron and the heavy-hole levels. The transition energies scale linearly with the As composition in each QW (fig. 2-b).By scanning across the etched stripes, we could determine the local changes of the spectral parameters for each QW line as a function of the laser beam position. The first observation is that the etching processes do not introduce any spectral broadening of the PL lines. In fact, for some of the etching processes evaluated, a sharpening of the lines is even observed. This is in contrast with the general assumption that dry etching produces some disorder-induced PL line broadening. The line positions are almost not affected by the etching processes. Finally, we observe some changes in the intensities of the QW lines across the stripes: these intensities decrease strongly near the edges. The different etching processes induce a different magnitude for this effect. These trends will be analyzed in terms of the modifications introduced within the etched material in the area close to the stripe edges. In some cases, the lateral extension of these modifications can reach 10 µm or more.[1] J. P. Landesman, N. Isik-Goktas, R. R. LaPierre, C. Levallois, S. Ghanad-Tavakoli, E. Pargon, C. Petit-Etienne and J. Jiménez, J. Phys. D: Appl. Phys. 54, 445106 (2021)

Low-temperature spatially-resolved luminescence spectroscopy of microstructures with strained III-V quantum wells

International audienceThe continuous development of advanced photonic devices based on 3-dimensional structuring of active materials calls for more efforts on characterization techniques. This statement applies, in particular, to processes, such as dry etching applied to III-V semiconductors. Dry (or plasma-based) etching is frequently used within nanofabrication platforms to realize semiconductor lasers, ridge waveguides, photonic integrated circuits, etc. Luminescence techniques (photo-luminescence or cathodo-luminescence) can analyze the material’s properties after dry etching, especially for direct band-gap semiconductors. The presence of non-radiative defects, changes in the local stress/strain, etc., can be probed in detail owing to the spatial resolution of this class of techniques, and their quantification can also be addressed. Our goal in this work is to highlight a robust methodology involving the design of specific test materials, including a series of different quantum wells (QWs) located at well-defined depths below the surface. The spectroscopic signature of these QWs provides valuable information, which can be used to assess the changes occurring to the III-V semiconductor material due to dry etching.The test structures have been designed on (100)-oriented InP substrates, with alternating InAsxP1-x QWs and InP barriers. Sequences of typically 8 QWs with graded As composition were grown, with a fixed thickness (7 to 8 nm), separated by 100 nm InP barriers. The shallowest QW is located 300 nm below the sample surface. By grading the As composition in the QWs (with x typically between 0.35 and 0.5), the luminescence signal for each QW could be unambiguously identified with sharp lines when measured at very low temperatures. These structures were grown by gas-source molecular beam epitaxy. Figure 1 illustrates such a structure.In a second step, a SiNx film was deposited by plasma-enhanced chemical vapor deposition and patterned using standard optical lithography. Elongated stripes were thus defined, whose width varies between 1 and 50 µm, and length is a few mm. Finally, plasma etching was used to fabricate stripes within the QW structures, using this SiNx film as a hard mask. Based on Cl2/CH4/Ar and H2/CH4/Ar, different gas mixtures were employed to perform this etching.We have characterized these etched stripes by micro-PL at 10 K in a specially designed optical cryostat, allowing a spatial resolution of approximately 1 µm and a step-size of 5 µm for mapping the luminescence signal. A 1064 nm laser source was chosen for the excitation of the PL signal in our samples. The choice of this wavelength allows selective excitation of the QWs, avoiding excitation of the InP barrier material. As shown in figure 2, the spectrum displays well-identified lines, which can be attributed clearly to the different QWs in the sample. The very sharp lines (full width at half maximum of the order of 4 meV) attest to the very high sample quality. A fitting procedure was implemented to determine the spectral characteristics of each transition, as illustrated by the red line (“Model”) in fig. 2-a. We have established ([1]) that the observed PL lines are associated with single optical transitions in each QW between the electron and the heavy-hole levels. The transition energies scale linearly with the As composition in each QW (fig. 2-b).By scanning across the etched stripes, we could determine the local changes of the spectral parameters for each QW line as a function of the laser beam position. The first observation is that the etching processes do not introduce any spectral broadening of the PL lines. In fact, for some of the etching processes evaluated, a sharpening of the lines is even observed. This is in contrast with the general assumption that dry etching produces some disorder-induced PL line broadening. The line positions are almost not affected by the etching processes. Finally, we observe some changes in the intensities of the QW lines across the stripes: these intensities decrease strongly near the edges. The different etching processes induce a different magnitude for this effect. These trends will be analyzed in terms of the modifications introduced within the etched material in the area close to the stripe edges. In some cases, the lateral extension of these modifications can reach 10 µm or more.[1] J. P. Landesman, N. Isik-Goktas, R. R. LaPierre, C. Levallois, S. Ghanad-Tavakoli, E. Pargon, C. Petit-Etienne and J. Jiménez, J. Phys. D: Appl. Phys. 54, 445106 (2021)