Growth of InAsSb/InAs MQWs on GaSb for mid-IR photodetector applications

We report the OMVPE growth and characterization of InAsSb/InAs strain balanced multiple quantum wells lattice-matched to GaSb substrates for potential application as mid-infrared detectors for wavelengths beyond 4 μm$\mathrm{\mu }\mathrm{m}$. Detailed transmission electron microscopy measurements were performed to evaluate the degree of Ga and Sb intermixing at the GaSb/InAsSb and InAs/InAsSb interfaces. Photoluminescence emission up to 5 μm$\mathrm{\mu }\mathrm{m}$ was observed for superlattice structures with only 15% antimony. The dependence of PL on wavelength is red shifted compared to expectations based on type I band alignment.     

Microstructure of interfacial HgTe/CdTe superlattice layers for growth of HgCdTe on CdZnTe (2 1 1)B substrates

Transmission electron microscopy has been used to characterize the microstructure of HgTe/CdTe superlattices (SLs) grown by molecular beam epitaxy on CdZnTe(2 1 1) B substrates. The purpose of these intermediate layers was to improve the quality of subsequent HgCdTe (MCT) epilayers intended for infrared detectors. The observations confirmed that the SLs smoothed out the surface roughness of the substrate, and showed that threading dislocations were prevented from reaching the MCT epilayers. High-quality growth of MCT on CdZnTe using the HgTe/CdTe interfacial layers has been demonstrated.     

Growth and characterization of GaSb/AlGaSb multi-quantum well structures on Si (0 0 1) substrates

GaSb/AlGaSb multi-quantum well (MQW) structures with an AlSb initiation layer and a relatively thick GaSb buffer layer grown on Si (0 0 1) substrates were prepared by molecular beam epitaxy (MBE). Transmission electron microscopy (TEM) images and high-resolution X-ray diffraction (XRD) patterns indicated definite MQW structures. The photoluminescence (PL) emission around 1.55 μm wavelength was observed for 10.34 nm GaSb/30 nm Al_0.6Ga_0.4Sb MQW structure at room temperature. Dependence of PL emission energy on GaSb well width was well explained by finite square well potential model.     

Electrical properties of bulk amorphous semiconductor (GaSb)38Ge24

Temperature dependences of the dc conductivity and thermopower of a (GaSb)₃₈Ge₂₄ homogeneous bulk amorphous alloy are investigated at 110–425 K and at 180–400 K, respectively. The samples were prepared by spontaneous solid-state amorphization of a quenched crystalline high-pressure phase heated from 77 to 430 K at ambient pressure. In contrast to the parent amorphous GaSb compound exhibiting an unusual combination of electrical properties, amorphous (GaSb)₃₈Ge₂₄ is found to be a typical p-type semiconductor well described by the conventional Mott–Davis model.     

N-type doping of GaN/Si(1 1 1) using Al0.2Ga0.8N/ALN composite buffer layer and Al0.2Ga0.8N/GaN superlattice

The characteristics of Si-doped and undoped GaN/Si(1 1 1) heteroepitaxy with composite buffer layer (CBL) and superlattice are compared and discussed. While as-grown Si-doped GaN/Si(1 1 1) heteroepitaxy shows lower quality compared to undoped GaN, crack-free n-type and undoped GaN with the thickness of 1200 nm were obtained by metalorganic chemical vapor deposition (MOCVD). In order to achieve the crack-free GaN on Si(1 1 1), we have introduced the scheme of multiple buffer layers; composite buffer layer of Al_0.2Ga_0.8N/AlN and superlattice of Al_0.2Ga_0.8N/GaN on 2-in. Si(1 1 1) substrate, simultaneously. The FWHM values of the double-crystal X-ray diffractometry (DCXRD) rocking curves were 823 arcsec and 745 arcsec for n-GaN and undoped GaN/Si(1 1 1) heteroepitaxy, respectively. The average dislocation density on GaN surface was measured as 3.85×10⁹ and 1.32×10⁹ cm⁻² for n-GaN and undoped GaN epitaxy by 2-D images of atomic force microscopy (AFM). Point analysis of photoluminescence (PL) spectra was performed for evaluating the optical properties of the GaN epitaxy. We also implemented PL mapping, which showed the distribution of edge emission peaks onto the 2 inch whole Si(1 1 1) wafers. The average FWHMs of the band edge emission peak was 367.1 and 367.0 nm related with 3.377 and 3.378 eV, respectively, using 325 nm He-Cd laser as an excitation source under room temperature.     

Effect of hetero-interfaces on in situ wafer curvature behavior in InGaAs/GaAsP strain-balanced MQWs

Using high-accuracy in situ curvature measurement during growth of InGaAs/GaAsP strain-compensated multiple quantum wells (MQWs) by metal organic vapor phase epitaxy (MOVPE), we have successfully clarified the effect of hetero-interfaces on strain control in InGaAs/GaAsP strain-balanced MQWs. By analyzing curvature transients and X-ray diffraction (XRD) fringe patterns, we found that an inadequate gas-switching sequence induces unintended atomic content at the interfaces between InGaAs and GaAsP and then influences the average strain of the structure. Through considering the atomic characteristics and measuring the reflectance anisotropy transient during growth, it has been revealed that the optimized stabilization time for arsenic and phosphorus mixture before GaAsP barrier growth should be longer than 3 s at 610 °C.     

Structural and magnetic properties of Ni2MnIn Heusler thin films grown on modulation-doped InAs heterostructures with metamorphic buffer

This paper reports on the morphological, structural, magnetic, and magneto-optic properties of Ni₂MnIn Heusler films grown on InAs-high electron-mobility transistor structures (HEMT) with metamorphic buffers for spintronic applications. Similar to our previous results on the Ni₂MnIn/InAs (001) system, the Heusler layer is found to have a (110) orientation relative to the (001) InAs-HEMT surface. We observe almost equal spin-polarizations for Heusler films on (001) InAs-HEMT as well as on (001) InAs. In addition, we find further support for interfacial intermixing previously reported for the Ni₂MnIn/InAs (001) system. On the other hand, the Heusler/InAs-HEMT system shows distinct morphologic, structural, and magnetic properties as compared to the Ni₂MnIn/InAs (001) system. In particular, more rapid and complex plastic deformation effects resulting in a high surface density of pin-holes in the Heusler films are found. We report on complex mutual deformation effects between the Heusler films and the underlying InAs-HEMT structure. Furthermore, a hysteresis loop squareness close to 1 for a 50 nm Heusler film on InAs-HEMT is observed. We tentatively associate these phenomena with the higher mismatch strain of the Ni₂MnIn/InAs-HEMT system compared to Ni₂MnIn films grown on (001) InAs.     

Controlling the formation of quantum dot pairs using nanohole templates

Modifying the shape of nanoholes formed by arsenic debt epitaxy by the overgrowth of a thin GaAs buffer is shown to provide a simple and robust method to grow low density lateral In(Ga)As quantum dot pairs (QDPs). We present here a systematic study of the effect of GaAs buffer thickness, InAs deposition amount, substrate temperature and arsenic overpressure on dot nucleation and QDP formation. A (10–30) nm GaAs buffer over nanoholes initially $～10.5\mathrm{nm}$ deep, (60–80) nm wide results in up to 80% of the nanoholes containing QDPs. The QD pairs are aligned along the [110] direction and have centre-to-centre separation of $～38\mathrm{nm}$. These QDPs form following InAs deposition between 1.3 ML and 1.6 ML at 490 °C under an arsenic arrival flux of 0.6 ML/s. From the infilling of the hole prior to QD formation, we estimate a net indium surface flux towards the hole of $～7$ times the incident flux. The substrate temperature does not significantly alter the dot distribution over the range (470–510) °C. However, the QDP formation is very sensitive to the arsenic overpressure over the range (0.6–1.2) ML/s because of a partial collapse of the nanohole, due to mass transport as the substrate passes through the (2×4) to c(4×4) surface reconstruction around 500 °C.     

Study of multiple-stacking growth of 1.55 μm InAs columnar quantum dots with modulated tensile-strained InGaAsP barrier layers

An investigation was performed of columnar InAs quantum dots (CQDs) with modulated tensile-strained InGaAsP barriers in which the amount of tensile strain in the upper parts was higher than in the lower parts, the dots being deposited on an InP substrate grown by metalorganic vapor phase epitaxy. The smaller tensile strain of the barrier layers in the lower parts made the photoluminescence (PL) wavelength longer while the larger tensile strain of the barrier layers in the upper parts increased the strain compensation of the CQDs. Compared to CQDs with uniformly tensile-strained barriers, 1.55 μm emission was obtained at a higher average strain of barrier layers. By utilizing modulated tensile-strained barriers, triple-stacking of 12-fold CQDs with a PL wavelength of 1.55 μm using 30-nm-thick spacer layers was achieved with good crystallinity, indicating suitability for fabrication of high density CQDs.     

Arsenic cross-contamination in GaSb/InAs superlattices

We have investigated the cross-contamination of As in GaSb/InAs superlattices. We demonstrate a method of varying the lattice constant of the superlattice. By controlling the As background pressure in the growth chamber, the strain can be controlled to about 0.01%, corresponding to As cross-incorporation variations of about ±1%. The distribution of As is investigated by X-ray diffraction and cross-sectional scanning tunneling microscopy, and the critical thickness is obtained.     

Low-density InAs QDs with subcritical coverage obtained by conversion of In nanocrystals

We report growth of InAs/GaAs quantum dots (QDs) by molecular beam epitaxy with low density of 2 μm^?2 by conversion of In nanocrystals deposited at low temperatures. The total amount of InAs used is about one monolayer, which is less than the critical thickness for conventional Stranski–Krastanov QDs. We also demonstrate the importance of the starting surface reconstruction for obtaining uniform QDs. The QD emission wavelength is easily tunable upon post-growth annealing with no wetting layer signal visible for short anneals. Microphotoluminescence measurements reveal well separated and sharp emission lines of individual QDs.     

Selective area molecular beam epitaxy of InAs on GaAs (110) masked substrates for direct fabrication of planar nanowire field-effect transistors

We have synthesized InAs nanowires (NWs) by selective area molecular beam epitaxy (SA-MBE) on GaAs masked substrates. In particular, we have obtained in-plane-oriented NWs on the (110) plane, and then directly applied the NWs to planar nanowire field-effect transistors (NWFETs) using conventional electron beam lithography without a NW dispersion process. We have measured output and transfer characteristics of the NWFETs at room temperature, and obtained a current swing but no turning off, and a field-effect mobility peak of 150 cm²/V-s. We have also observed almost no temperature influence on field-effect mobility between 2 K and 300 K, suggesting a high-dense surface accumulation layer even at low temperatures.     

Polarities of AlN films and underlying 3C-SiC intermediate layers grown on (1 1 1) Si substrates

The hydride vapor phase epitaxy (HVPE) of {0 0 0 1} AlN films on {1 1 1} Si substrates covered with epitaxial {1 1 1} cubic SiC (3C-SiC intermediate layers) was carried out. 3C-SiC intermediate layers are essential to obtain high-quality AlN films on Si substrates, because specular AlN films are obtained with 3C-SiC intermediate layers, whereas rough AlN films are obtained without 3C-SiC intermediate layers. We determined the polarities of AlN films and the underlying 3C-SiC intermediate layers by convergent beam electron diffraction (CBED) using transmission electron microscopy. For the first time, the polarities of the AlN films and the 3C-SiC intermediate layers were determined as Al and Si polarities, respectively. The AlN films were hardly etched by aqueous KOH solution, thereby indicating Al polarity. This supports the results obtained by CBED. The result is also consistent with electrostatic arguments. An interfacial structure was proposed. The 3C-SiC intermediate layers are promising for the HVPE of AlN films on Si substrates.     

Growth of highly oriented crystalline α-Fe/AlN/Fe3N trilayer structures on Si(1 1 1) substrates by molecular beam epitaxy

We have realized highly oriented nitride-based α-Fe/AlN/Fe₃N ferromagnetic hybrid structures on Si(1 1 1) substrates by molecular beam epitaxy using AlN/SiC intermediate layers. A two-step hysteresis loop, typically observed in magnetic tunneling junctions, was clearly observed in magnetization versus magnetic field measurements. This result indicates the formation of ferromagnetic α-Fe and Fe₃N layers separated by 8-nm-thick AlN layers over approximately 1 cm² area, and also shows the difference in coercive field between the two ferromagnetic layers.     

Self-alignment of self-assembled InAs quantum dots

The lateral self-alignment properties of self-assembled InAs quantum dots (QDs) on a conventional GaAs (1 0 0) substrate by molecular beam epitaxy were investigated. The shape and optical properties of QDs were investigated by atomic force microscopy, transmission electron microscope, and photoluminescence (PL). Attempts were made to grow InAs-QDs using the In-interruption growth technique, in which the In flux was periodically interrupted. QDs grown without using the In-interruption growth technique were grown randomly on all regions. On the other hand, in the case of QDs grown using the In-interruption growth technique, QDs were self-aligned at the boundary between bright and dark regions, the PL intensity was increased and the PL peak position of QDs were red-shifted to 1300 nm. This represent a new technique for growing self-aligned QDs because no extra processing such as electron-beam lithography, V-grooves and surface modification by scanning tunneling microscopy is needed, and aligned QDs can be grown in situ on conventional GaAs substrates.     

Critical thickness of the 2-dimensional to 3-dimensional transition in GaSb/GaAs(001) quantum dot growth

The phase transformation from planar to quantum dot growth is driven by strain energy reduction at the cost of surface energy. By calculating and comparing the strain energies of monolayer thick GaSb and InAs films on GaAs(001), a critical thickness for the 2-dimensional to 3-dimensional phase transformation of about 1.2 ML was derived for the GaSb/GaAs quantum dot system. This value is in agreement with the direct observation of the effectively deposited amount of material using cross-sectional scanning tunneling microscopy. Deviating experimental literature values can be traced back to the neglect of the Sb-for-As exchange process.     

Growth kinetics and boron doping of very high Ge content SiGe for source/drain engineering

We have studied the in-situ boron doping of high Ge content Si_1?xGe_x layers (x=0.3, 0.4 and 0.5). These layers have been grown at low pressure (20 Torr) and low temperature (600–650 °C) with a heavily chlorinated chemistry on blanket Si(0 0 1) substrates. Such a chemistry yields a full selectivity versus SiO₂ (isolation) and Si₃N₄ (sidewall spacers) on patterned wafers with gate stacks. We have quantified the impact of the diborane flow on the SiGe layer crystalline quality, its resistivity, the SiGe:B growth rate and the apparent Ge concentration. Resistivity values lower than 1 mΩ cm are easily achieved, all the more so for high Ge content layers. The SiGe growth rate increases and the apparent Ge concentration (from X-ray diffraction) decreases as the diborane flow increases. B atoms (much smaller than Si or Ge) indeed partially compensate the compressive strain in the SiGe:B layers. We have also probed the in-situ boron and phosphorus doping of Si at 750 °C, 20 Torr with a heavily chlorinated chemistry. The B ions concentration increases linearly with the diborane flow, then saturates at a value close to 4×10¹⁹ cm^?3. By contrast, the P ions concentration increases sub-linearly with the phosphine flow, with a maximum value close to 9×10¹⁸ cm^?3. Adding diborane (phosphine) to the gaseous mixture leads to a sharp increase (decrease) of the Si:B (the Si:P) growth rates, which has to be taken into account in device layers. All the know-how acquired will be most handy for the formation of in-situ doped recessed or raised sources and drains in metal-oxide semiconductor devices.     

Comparison of growth methods for Si/SiO2 nanostructures as nanodot hetero-emitters for photovoltaic applications

Two different growth mechanisms are compared for the fabrication of Si/SiO₂ nanostructures on crystalline silicon (c-Si) to be used as hetero-emitter in high-efficiency solar cells: (1) The decomposition of substoichiometric amorphous SiO_x (a-SiO_x) films with 0 < x < 1.3 and (2) the dewetting of thin amorphous silicon (a-Si) layers.The grown layers are investigated with regard to their structural properties, their passivation quality for c-Si wafer substrates and their electrical properties in order to evaluate their suitability as a nanodot hetero-emitter. While by layer decomposition, no passivating nanodots could be formed, the dewetting process allows fabricating nanodot passivation layers at temperatures as low as 600 °C. The series resistance through Ag/[Si-nanodots in SiO₂]/c-Si/Al structures for dewetting is similar to nanostructured silicon rich SiO_x films. Still, a nanodot hetero-emitter which exhibits both a satisfying passivation of the substrate and induces a high band bending by doping at the same time could not be fabricated yet.     

Deposition of phosphorus doped a-Si:H and μc-Si:H using a novel linear RF source

A roll-to-roll PECVD system for thin film silicon solar cells on steel foil has been developed by ECN in collaboration with Roth and Rau AG. It combines MW–PECVD for fast deposition of intrinsic Si and novel linear RF sources, which apply very mild deposition conditions, for the growth of doped Si layers. The RF and MW sources can be easily scaled up to deposition widths of up to 150 cm. Here, we report on n-type doping, achieved by RF–PECVD from a H₂/SiH₄/PH₃ mixture in the reaction chamber. The best n-type a-Si:H layers showed E_act = 0.27 eV and σ_d = 2.7 × 10^?3 S/cm. Also thin layers down to 20 nm were of device quality and were deposited at a rate of 0.4 Å/s. Furthermore, n-type μc-Si:H layers with thicknesses of 150 nm, with E_act = 0.034 eV and σ_d = 2 S/cm were grown. Good quality n-type μc-Si:H layers can be made for layer thicknesses down to 50 nm at a rate of 0.15 Å/s. To conclude, the novel RF source is well-suited for the growth of n-doped a-Si:H and μc-Si:H layers for roll-to-roll solar cell production.     

Growth of thick,continuous GaN layers on 4-in. Si substrates by metalorganic chemical vapor deposition

We report on the growth of thick GaN epilayers on 4-in. Si(1 1 1) substrates by metalorganic chemical vapor deposition. Using intercalated AlN layers that contribute to counterbalance the tensile strain induced by the thermal mismatch between gallium nitride and the silicon substrate, up to 6.7 μm thick crack-free group III-nitride layers have been grown. Root mean-squares surface roughness of 0.5 nm, threading dislocation densities of 1.1×10⁹ cm^?2, as well as X-ray diffraction (XRD) full widths at half-maximum (FWHM) of 406 arcsec for the GaN(0 0 2) and of 1148 arcsec for the GaN(3 0 2) reflection have been measured. The donor bound exciton has a low-temperature photoluminescence line width of 12 meV. The correlation between the threading dislocation density and XRD FWHM, as well as the correlation between the wafer curvature and the GaN in-plane stress is discussed. An increase of the tensile stress is observed upon n-type doping of GaN by silicon.