Sidewall Facets Research Articles

Photoluminescence properties of 13×13 nm GaAs quantum wires buried in trench structures reduced by growing GaAs/AlAs superlattice layers

We report lateral-size control of GaAs/AlAs trench-buried quantum wires (QWRs) on a scale of 10 nm by metalorganic chemical vapor deposition using nonplanar substrates. The lateral width is reduced to 12–13 nm by growing GaAs/AlAs superlattice layers (SLs) on the (110) sidewall facets of the trenches, where the roughness of the SL sidewalls is approximately several monolayers. Low-temperature photoluminescence (PL) properties of nearly square 13×13 nm QWRs buried in the trenches exhibit a strong PL blue shift of 85 meV with respect to the band-gap energy of GaAs bulk and PL polarization anisotropy of 25% due to two-dimensional quantum confinement effects. The emission from Ga-rich AlGaAs regions in the trenches constituting AlGaAs vertical quantum wells was also observed. We demonstrate that substituting GaAs/AlAs SLs for the AlGaAs layer effectively eliminates the undesired emission levels caused by the inevitable Al content fluctuation in the AlGaAs layer grown on nonplanar structures.

Novel Formation Method of Quantum Dot Structures by Self-Limited Selective Area Metalorganic Vapor Phase Epitaxy

We have fabricated AlGaAs/GaAs quantum dot structures using selective area metalorganic vapor phase epitaxy (MOVPE). First, GaAs pyramidal structures with fourfold symmetric {011} facet sidewalls are formed on SiNx-masked (001) GaAs with square openings. Once the pyramidal structures were completely formed, no growth occurs on the top or sidewalls of the pyramids. Furthermore, the shape and width of the top area observed using a scanning electron microscope (SEM) and an atomic force microscope (AFM) is shown to be highly uniform. This indicates that self-limited growth occurs. Next, using these uniform pyramids, GaAs quantum dots are overgrown on top of the pyramids under different growth conditions. Sharp photoluminescence (PL) spectra are observed from uniform quantum dots.

The selective epitaxial growth of silicon

The techniques of selective silicon growth (SSG) have already found many applications in device fabrication in respect of both process simplification and the development of novel device structures; the techniques involve selective epitaxial growth (SEG), epitaxial lateral overgrowth (ELO), selective polycrystalline Si growth (SPD) and selective polycrystalline-epitaxial growth (SPEG or SSPD). The basic technique involves substrate preparation to produce the necessary seed windows in an insulating layer together with the subsequent removal of surface damage ( e.g. from a reactive ion etch), ex-situ and in-situ cleaning processes and the selective growth itself. Ex-situ processing generally involves combinations of a wet chemical ( e.g. RCA) clean to remove organics and/or metallics, HF treatment to remove oxide and (hydrogen) passivate the Si surface against reoxidation and/or UV-ozone (UVOC) exposure designed to remove carbon and to provide a thin passivating oxide. The in-situ clean can be a combination of an HF-vapour treatment providing a similar surface to the above, UVOC providing the same treatment as above (but with the production of a relatively thinner passivating oxide which is more easily desorbed at low temperatures), followed by some sort of plasma clean or a H 2 bake to remove the surface oxide prior to growth; however, depending upon the particular reactor system being used, an in-situ “pre-bake” may not be necessary if operating under ultrahigh vacuum conditions. The ex-situ and in-situ cleaning procedures should provide substrate surfaces adequate for SSG with minimal undercutting of the field area mask. The growth process involves optimization of temperature, pressure and gas flows to obtain full control over selectivity ( i.e. polycrystalline Si nucleation on the field), side-wall faceting and defect generation and must also consider autodoping effects for device applications. This paper will review the techniques of surface preparation and SSG and discuss the results obtained from a study of SEG and ELO for advanced bipolar applications, using a modified Applied Materials 7811 epitaxial reactor system.

Selective embedded growth by LP-MOVPE in the Ga-In-As-P system

We have investigated the selective embedded growth by LP-MOVPE (low pressure metalorganic vapor phase epitaxy) of InP and GaInAs (on InP) and of GaAs and GaInP (on GaAs) in grooves with (011), (111)A and (111)B side wall facets prepared by wet chemical etching and reactive ion etching of (100) substrates. For all materials studied a reduction of the growth rates on the side walls compared to those on the bottom was observed. This reduction is enhanced by surface migration of nutrients from side wall facets to the bottom of the groove. The deposition rate is also affected by the geometry of the trenches. For narrow lines this results in a drastically reduced rate compared to that in wider trenches. Additionally, the composition of the selectively grown ternaries GaInAs and GaInP is different from that in large area growth and depends further on the size of the grooves. Based on the insights gained in this study grooves, refilled with GaInAs/InP or GaInP/GaAs multilayers with planar surfaces, could be realized. In contrast to the growth of GaInP on GaAs and of InP on InP, the deposition of the arsenic-containing III–V compounds on the side wall facets of the grooves can be successfully suppressed.

MOCVD methods for fabricating GaAs quantum wires and quantum dots

Semiconductor low-dimensional structures such as quantum wires and quantum dots fabricated by metalorganic chemical vapor deposition (MOCVD) are reported. GaAs/AlAs quantum wire arrays including fractional-layer superlattices were grown on (001) GaAs vicinal surfaces. Uniform lateral superlattices were observed by transmission electron microscopy. However, polarization-dependent photoluminescence and the optical absorption spectra suggest that significant alloying occurs in AlAs/GaAs vertical interfaces. GaAs tetrahedral quantum dots buried in AlGaAs were also fabricated on a SiO 2 masked (111)B GaAs substrate partially etched to a triangular shape. First, AlGaAs truncated tetrahedral structures with three {110} facets were grown on GaAs triangular areas. Next, GaAs dot structures were sequentially grown on top of the AlGaAs truncated tetrahedron. Finally, AlGaAs was over-grown on {110} sidewall facets with a different growth condition. We observed large quantum size effects for about 100 nm TQD in low temperature photoluminescence, which is in good agreement with calculations.

Deposition by LP-MOVPE in the Ga-In-As-P system on differently oriented substrates

We have studied the MOVPE of GaInAs and its binary constituents GaAs and InP and its binary constituents GaP and InP on InP, and on GaAs (100), (011), (111)A, (111)B oriented unmasked planar substrates in order to gain an understanding of the deposition on the bottom plane and on side wall facets of trenches in selective embedded growth. Temperature (940 K) and total pressure (20 hPa) were selected with this purpose in mind. It is shown that the behaviour of the binaries provides an understanding of the factors determining the deposition of the ternaries and that insights for selective growth on partially masked substrates can be gained from these data. In contrast to InP and InAs growth, the rates for the Ga binaries GaAs and GaP and the ternaries depend on the substrate orientation. The variation of the rates of the ternaries is very similar to that of the binary Ga constituents. More importantly, also the composition (Ga content) of the ternaries appears to track with this parameter and varies rather widely, particularly for GaInAs. In the case of selective embedded deposition the differences in rates between the different surfaces tend to be enhanced compared to those on unmasked substrates due to migration effects.

GaAs tetrahedral quantum dots grown by selective area MOCVD

GaAs tetrahedral quantum dots (TQDs) buried in AlGaAs were fabricated using selective area metalorganic chemical vapor deposition (MOCVD). The substrates were SiO 2 masked (111)B GaAs, which were partially etched free of SiO 2 over triangular areas using electron beam lithography and reactive ion etching techniques. First, truncated tetrahedral AlGaAs buffer layers with {110} facet sidewalls were grown over the triangular areas. Next, GaAs TQDs were sequentially grown on top of the AlGaAs. Finally, AlGaAs layers were overgrown on the resulting tetrahedral structures. The height of the GaAs tetrahedrons was estimated to be 20 nm. Photoluminescence of GaAs TQDs buried in AlGaAs was measured at 8.5 K. A clear emission peak from GaAs TQDs was observed at 810 nm. The energy shift from the GaAs emission peak is 19meV, which agrees well with the calculation.

GaAs Quantum Dots by MOCVD

ABSTRACTNew GaAs quantum dots called tetrahedral quantum dots (TQDs) were fabricated using selective area metalorganic chemical vapor deposition (MOCVD). GaAs sub-micron crystals were completely buried in AlGaAs with single growth run without any processing damage at heterojunction interface. The substrates were SiO2 masked (111)B GaAs, which are partially etched free of SiO2 over triangular area using electron beam lithograpy and reactive ion etching techniques. First, truncated tetrahedral AlGaAs buffer layers with {110} facet sidewalls were grown in triangular area. Next, GaAs TQDs were sequentially grown on the top of AlGaAs. Finally, AlGaAs layers were overgrown on the resulting tetrahedral structures. The shape of GaAs tetrahedron was measured by an atomic force microscope(AFM). The size of bottom triangle of GaAs TQDs were estimated to be 20 nm. The size fluctuation was about 2%, which means that uniformity of selective area growth is excellent. Photoluminescence of GaAs TQDs buried in AlGaAs was measured at 8.5K. A clear emission peak from GaAs TQDs was observed at 810 nm. The energy shift from the GaAs emission peak is 19meV, which agrees well with the calculation. The results suggest that the selective area MOCVD method is very promising to fabricate GaAs quantum dots.

THE SELECTIVE EPITAXIAL GROWTH OF SILICON

The SEG technique and its extension epitaxial lateral overgrowth (ELO) have already found many applications in device fabrication both in terms of process simplification and new device structures. The basic processes involve substrate preparation, ex-situ cleaning, in-situ surface oxide removal and SEG/ELO growth. Substrate preparation involves fabrication of the necessary seed-windows in, e.g. oxide or nitride layers down to the single crystal substrate : this is usually achieved using a reactive ion etch (RIE) process followed by the growth and subsequent etching of a sacrificial oxide (SO) for surface damage removal. Ex-situ cleaning usually consists of a wet chemical treatment, e.g. RCA, with the final RCA-2 step leaving the silicon surface coated with a reproducible thin oxide layer which also serves to remove various contaminants. The in-situ clean removes the latter oxide through a low pressure H2 prebake, using a temperature which effectively etches the oxide via the [MATH] disproportionation reaction, but which minimises the concomitant undercutting of the exposed SiO2/Si oxide - substrate interface by the same reaction. The growth process involves optimisation of the main parameters, i.e. temperature, pressure and gas flows, to obtain full control of selectivity (i.e. polysilicon nucleation on the field area), sidewall faceting, defect generation and autodoping. This paper will review the technique of selective silicon epitaxy and discuss the results obtained during the optimisation of the above parameters for device applications using a modified Applied Materials LPCVD vertical barrel epitaxial reactor system.

Model for facet and sidewall defect formation during selective epitaxial growth of (001) silicon

An atomistic growth model is used to explain sidewall facet and defect formation during selective epitaxial growth of (001) silicon. Films grown through oxide windows with {110} sidewall orientations exhibit facets (typically {311} planes) adjacent to the sidewall. This region also has a high density of twins. Films grown in windows oriented to have {100} sidewalls have no sidewall facets and a very low defect density. The facet morphology and twin formation at {110} sidewalls are both explained by the influence of the oxide on nucleation of {111} planes. Similar considerations indicate that films grown along {100} sidewalls are less susceptible to facet and defect formation, as observed. Experimental data on film morphology and defect structure are used to support the model.