Substrate-epilayer Interface Research Articles

Proximity Effects of the Vacuum and Substrate on the Behavior of Ultrathin Epilayers

The present considerations are motivated by (i) the need to grow thin films with perfection in crystallinity and thickness uniformity, both of importance for purposes of device fabrication and the study of two-dimensional systems, and (ii) by the importance to understand the roles of vacuum and substrate proximity effects during the ultrathin growth stage, as these effects may be decisive in tailoring the final product. It is accepted (a) that the need is best served by growing epitaxially—a substrate proximity phenomenon—and (b) that the quality of epilayers depends greatly on the mode of misfit accommodation at the epilayer-substrate interface and on the mode of growth. In classical theoretical analyses of epitaxy, these modes are modeled in terms of the interfacial lattice misfit, the amplitudes of lateral variation of interfacial atomic interaction, defect energies of the bicrystal and the elastic properties of the epilayer. The aim of this paper is to report on and search for perspective of attempts to quantify—using embedded-atom method potentials—the effects of vacuum and substrate proximities on these modeling parameters. The objective is to focus attention on the fact that the ultrathin film values of these parameters may be significantly different from their bulk values, which have been employed in the past for predictive purposes.

Iodine use in solid-source III–V molecular-beam epitaxy

Use of iodine for in situ etching of GaAs, AlAs, and InAs in solid-source molecular-beam epitaxy has been explored. Reflectance high-energy electron-diffraction intensity oscillations have been observed during iodine etching of GaAs and InAs, indicating a molecular layer-by-layer nature of material removal. Etch rates of GaAs and InAs determined from the period of the oscillations are comparable for a given iodine flux. Secondary-ion-mass spectroscopy depth profiles indicate that the etching of AlAs is negligible, and that iodine can be used as a selective etch which stops on AlAs or AlGaAs layers. Electrical properties of GaAs layers grown with an iodine beam impinging on the surface are comparable to those of the layers grown without iodine. Use of iodine for the surface cleaning of GaAs has also been examined. Our results show that iodine etching prior to growth reduces the level of carbon contamination at the substrate epilayer interface.

Effect of the oxide-desorption temperature on the substrate–epilayer interface charge in organometallic vapor-phase epitaxy of GaAs

We found that the use of relatively low temperatures during oxide desorption in organometallic vapor-phase epitaxy of GaAs reduced the epilayer–substrate interface charge. No differences in Hall mobility were found between the samples grown with 550 and 650 °C desorption. O, C, Si, and S were the dominant impurities at the interface. Si and S, which are shallow donors, induce the interface-charge accumulation. Low-temperature desorption leaves oxygen at the surface, suppressing the activation of shallow dopants. A high level (≳1×1012/cm2) of Cl was detected on every GaAs substrate we measured, but its effect on epitaxy is not clear.

Effect of interfacial dopant layer on transport properties of high purity InP

Temperature dependent Hall measurements were used to investigate the role of unintentional interfacial dopant layers on the electrical properties of high purity InP. Secondary ion mass spectroscopy was used to measure the level of Si contamination present in several samples at the substrate-epilayer interface. We show that the presence of interfacial dopant gives rise to two layer conduction whose temperature dependence mimics the freezeout behavior expected for a deep donor. However, the magnetic field dependence of the Hall data at low temperatures shows the expected behavior for the two layer model, including an order of magnitude variation in the apparent sheet concentration and Hall mobility over the range from 0.1 to 0.6 T at 77 K. We show that the true bulk mobility and carrier concentration can be accurately determined in samples with high interfacial contamination by performing Hall measurements at several magnetic fields at 77 K.

Dynamics of bound-exciton luminescences from epitaxial GaN

Free- and bound-exciton luminescences of GaN epitaxial layers grown by a sublimation technique on 6H-SiC substrates were investigated using time-integrated and time-resolved photoluminescence measurements at low temperatures. Lifetimes were determined for the donor-bound exciton at 3.4722 eV and for two acceptor-bound excitons with energies of 3.4672 eV and 3.459 eV. On the basis of our results we obtain an upper limit of the free-exciton oscillator strength of 0.0046 for GaN. Luminescences between 3.29 eV and 3.37 eV are identified as due to excitons deeply bound to centers located near the substrate-epilayer interface. Free excitons are captured by these centers within 20 ps.

Diffuse x-ray scattering from misfit dislocations in SiGe epitaxial layers with graded Ge content

A theory has been developed describing x-ray diffuse scattering from misfit dislocations in epitaxial layers. This approach has been used for explaining the origins of diffuse x-ray scattering from SiGe layers with linearly graded Ge content. The distribution of the diffusely scattered intensity in reciprocal plane measured by triple-axis x-ray diffractometry has been compared with theoretical predictions and a good agreement has been achieved. It is demonstrated that the main part of the diffusely scattered intensity originates from random strains caused by misfit dislocations at the substrate–epilayer interface or in the relaxed part of the compositionally graded layers. The contribution of the threading dislocation segments to the diffuse scattering is rather small.

Modulation of excitonic reflectance at GaAs/GaAs interfaces

We have studied photoreflectance (PR) and reflectance spectra of undoped molecular-beam-epitaxy GaAs films on semi-insulating GaAs substrates at low temperature. In the PR spectra a sharp structure near the band-gap energy was observed which is sensitive to pump and probe light intensity. The origin of this structure was investigated by combining time-resolved and depth-resolved measurements. It is shown that the sharp PR structure arises from the superposition of a fast and a slow component of the modulated spectra. The slow PR signal originates from the epilayer–substrate interface. Both components are attributed to the modulation of exitonic transitions centered at slightly different energies.

On the development of misfit dislocation distributions in strained epitaxial layer interfaces

Electronic devices such as lasers with performance characteristics that are unique may be constructed by the growth of a thin layer of semiconductor epitaxially on a substrate. Since the materials are often selected for reasons other than a perfect match of lattice constants, the mismatch between the two layers produces strain in the epitaxial layer that has to be accommodated in some way. The two possibilities are firstly, that the layer is in perfect register with the substrate leaving a strain in the layer, or secondly, the strain can be partially relieved in the layer by an array of misfit dislocations in the substrate-epilayer interface. It has been shown that the distribution of dislocation spacings in samples with a dislocation density less than approximately 2 {times} 10{sup 4} cm{sup {minus}1} has a broad distribution, with a single peak and an extended tail. A second peak forms in the distribution as the dislocation density increases to approximately 1.5 {times} 10{sup 5} cm{sup {minus}1}. This peak is found initially at 1.5 times the first modal peak, and as the dislocation density increases further, the second peak increases in magnitude and shifts to 2 times the first modal peak. This second peak occursmore » when the dislocation density reaches approximately 2.5 {times} 10{sup 5} cm {sup {minus}1}. The authors have provided an explanation for why this should occur, suggesting that it is due to the formation of edge dislocations by the interaction of gliding 60{degree} dislocations.« less

Surface morphology, electrical and optical properties of gallium antimonide layers grown by liquid phase epitaxy

Liquid phase epitaxial (LPE) growth of gallium antimonide has been carried out employing equilibrium cooling, step cooling, supercooling and two phase solution growth techniques. An optimum temperature range of 500–550°C was found to be suitable for the growth of high quality layers. The morphology of layers grown by the first three techniques improved with increase in layer thickness. In contrast, better morphology was obtained for thin layers when grown from the two phase solution technique. While the equilibrium cooling technique gave a diffuse substrate-epilayer interface, sharp interfaces were obtained by the step cooling, supercooling, and two phase solution growth techniques. Photoluminescence spectroscopy and current-voltage measurements carried out on the grown layers revealed that the layers grown from Ga-rich melts exhibit superior optical and electrical properties as compared to those grown from Sb-rich melts.

Time Resolved Photoluminescence Spectroscopy on GaN Epitaxial Layers

AbstractFree and bound exciton luminescences as well as donor-acceptor pair recombination of GaN epitaxial layers on 6H-SiC and sapphire substrates were investigated using time integrated and time resolved photoluminescence measurements at low temperatures. Lifetimes are determined for the donor bound exciton at 3.4722eV and for two acceptor bound excitons with energies of 3.4672eV and 3.459eV. Luminescences between 3.29eV and 3.37eV are identified as due to excitons deeply bound to centers located near the substrate-epilayer interface.

Effects of storage time of epi-ready InP:Fe substrates on the quality of metalorganic vapour phase epitaxial grown InP

InP layers have been grown on epi-ready InP:Fe substrates by metalorganic vapour phase epitaxy (MOVPE). Hall measurements show an influence of the storage time of wafers on the electrical properties of the grown epitaxial layers. Hall data and electrochemical capacitance-voltage (ECV) profiles document an increase of donors at the substrate-epilayer interface for longer storage times. They also point to the existence of deep donors at the interface. The concentration of deep donors can be associated with the high oxygen level at the interface detected by secondary ion mass spectroscopy (SIMS). Ellipsometry proves that the oxide layer grows in thickness with time. Even after only four months of storage it was not possible to completely remove this oxide layer by thermal annealing under phospine.

Low temperature epitaxial growth by molecular beam epitaxy on hydrogen-plasma-cleaned silicon wafers

In a single step, the surfaces of silicon wafers were cleaned in an argon-hydrogen discharge plasma using the Balzers ultrahigh vacuum plasma source. The residual gas ions were monitored during the cleaning procedure to elucidate the process chemistry. Homoepitaxial growth by molecular beam epitaxy (MBE) at low temperatures could be achieved after the cleaning step, indicating a damage-free plasma procedure. Secondary ion mass spectrometry depth profiles of the substrate-epilayer interface show interface contamination levels of approximately 10 −2 monolayers for oxygen and less than 10 −3 monolayers for carbon. In additional experiments, patterned wafers were cleaned in the plasma. Local epitaxial growth by MBE at a temperature of 550°C avoiding high temperature steps could be achieved on these wafers. Furthermore, the influence of the plasma treatment on the wafer surface was investigated in detail for electron vs. bombardment and different bombardment energies using high-resolution X-ray rocking curve diffraction.

The nature of dislocation sources and strain relief in InAs yP 1− y films grown on 〈100〉 InP substrates

The nature of dislocation sources in a compressively strained InAs 0.48P 0.52 layer grown on a (100) InP substrate by gas source molecular beam epitaxy has been studied. Head's solution for an edge dislocation lying parallel to a free surface in an elastically isotropic, semi-infinite solid is used to evaluate the energetics of the one-dimensional dislocation arrays observed to form in the easy glide [01 1 ] direction prior to activation of sources which glide and cross-slip on {111} and {110}. The same solution is also used to derive the displacements of a misfit dislocation at the epilayer-substrate interface, and to characterize the origin of the diffraction contrast observed at the misfit dislocations in plan view TEM. Each source is shown to emit four sets of different dislocations. The effective stress acting on {111} and {110} during the first stages of activation of these dislocation sources has been determined.

Crystallography of epitaxial growth of wurtzite-type thin films on sapphire substrates

In this article, we present a crystallographic model to describe the epitaxial growth of wurtzite-type thin films such as gallium nitride (GaN) on different orientations of sapphire (Al2O3) substrates. Through this model, we demonstrate the thin films grown on (00⋅1)Al2O3 have a better epilayer-substrate interface quality than those grown on (01⋅2)Al2O3. We also show the epilayer grown on (00⋅1)Al2O3 are gallium-terminated, and both (00⋅1) and (01⋅2) surfaces of sapphire crystals are oxygen-terminated.

A model of internal lattice deformation of the interface during liquid phase heteroepitaxy of AxB1–xCyD1–y-type AIIIBV compounds

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Substrate orientation effects on dopant incorporation in InP grown by metalorganic chemical vapor deposition

We have investigated the doping incorporation and activation of InP growth using metalorganic chemical vapor deposition on <100≳, <311≳B, and <110≳ InP substrates. Effects of orientation, growth temperature, and V/III fluxes were studied. The dopants used were Zn from dimethylzinc [(CH3)2Zn] and diethylzinc [(C2H5)2Zn], S from hydrogen sulfide [H2S], Si from silane [SiH4], and Sn from tetraethyltin [(C2H5)4Sn]. The incorporation and activation of the p-type dopant Zn are elevated on the <311≳B and <110≳ planes, while the incorporation is suppressed for the n-type dopants (S, Si, and Sn). The n-type dopant Sn has similar incorporation and activation on the various substrate orientations studied. Anomalous Zn doping on the higher order planes <311≳B and <110≳ lead to the Zn incorporation exceeding the solubility limit in InP. Incorporated Zn levels as high as 1.0×1019 cm−3 were measured, and the corresponding activated Zn level was as high as 5.4×1018 cm−3 on a <110≳ InP substrate. Interdiffusion of the p-type dopant Zn into the S-doped n-type InP substrate is inhibited by a high S-doping level and segregates at the substrate–epilayer interface. If the S-doping level is lower than the Zn concentration, then Zn diffuses deep into the substrate at a uniform level.

Gallium and nitrogen ion implantation in MOVPE-grown ZnSe/GaAs

We report on n- and p-type doping of MOVPE-grown ZnSe layers on GaAs substrates by implantation with gallium or nitrogen and subsequent annealing. The samples were analyzed by Hall measurements, photoluminescence, Raman scattering and far-infrared reflectivity. For Ga implantation a free-carrier concentration in ZnSe of about n = 10 17 cm -3 is achieved. Furthermore, we observe that the annealing procedure leads to a considerable diffusion of Zn across the epilayer-substrate interface into the GaAs substrate, resulting in a highly p-type layer of about 1 μm thickness. This interface layer complicates the analysis of p-type ZnSe layers. However, nitrogen incorporation is evident from SIMS measurements and PL spectra.

In situ B-doped Si epitaxial films grown at 450‡ C by remote plasma-enhanced chemical vapor deposition: Physical and electrical characterization

In situ boron doping of Si epitaxial films grown at 450‡ C by remote plasma-enhanced chemical vapor deposition (RPCVD) has been studied using secondary ion mass spectroscopy (SIMS), Hall effect measurements, defect etching in conjunction with Nomarski microscopy, cross-sectional transmission electron microscopy (XTEM), and current-voltage measurements. Boron incorporation is shown to be controllable and electrically active from 7 × 1017 to over 1020 cm-3, with no dependence on process parameters (temperature, rf power, and substrate bias) in the ranges studied, other than the B2H6/SiH4 gas-phase ratio. No change in deposition rate upon introduction of B2H6 dopant gas is seen, contrary to what has been observed in several higher-temperature CVD processes. No defects such as stacking faults are seen under Nomarski microscopy, but a visible haze covers some areas ofin situ B-doped wafers. This haze appears to consist of amorphous cone-shaped structures with their apexes at the substrate-epilayer interface. The origin of the conical defects is believed to be related to some phenomenon at the initiation of growth. In order to evaluate the electrical quality ofin situ B-doped epilayers,P +/N mesa diodes have been fabricated using both homoepitaxial and heteroepitaxial (GexSi1-x)p-type epitaxial films. The electrical junction in these diodes coincides with the (epi-substrate)—interface in the grown films. To avoid interdiffusion or annealing effects during diode fabrication, all processing temperatures were kept at or below 450‡ C. Ideality factors are 1.2-1.3 for all diodes, indicating diffusion-limited transport rather than recombination in the depletion region.

Variations in the structural and electrical properties as a function of depth in CdTe layers grown on InSb substrates

The electrical properties of CdTe and CdMnTe layers grown on (001) InSb substrates by MBE have been studied as a function of depth through the layers. Using capacitance-voltage profiling techniques, the free carrier concentration has been found to be significantly reduced in the vicinity of the substrate and corresponding depth-resolved DLTS measurements have revealed the presence of trapping states only in the region of reduced carrier density. For Cd 1− x Mn xTe layers, a greater reduction in carrier density is observed as the Mn concentration is increased. These observations are explained in terms of the presence of extended defects which are expected to result from the relief of strain due to lattice mismatch at the substrate-epilayer interface. This interpretation is supported by measurements and computer-aided simulations of DCXRD rocking curves which indicate a graded strain structure in these layers.

Effects of S, Si, or Fe dopants on the diffusion of Zn in InP during MOCVD

Diffusion of Zn in InP during growth of InP epitaxial layers has been investigated in layer structures consisting of Zn-InP epilayers grown on S-InP and Fe-InP substrates, and on undoped InP epilayers. The layers were grown by metalorganic chemical vapour deposition (MOCVD) atT = 625° C andP = 75 Torr. Dopant diffusion profiles were measured by secondary ion mass spectrometry (SIMS). At sufficiently high Zn doping levels ([Zn] ≥8 × 1017 cm−3) diffusion into S-InP substrates took place, with accumulation of Zn in the substrate at a concentration similar to [S]. Diffusion into undoped InP epilayers produced a diffusion tail at low [Zn] levels, probably associated with interstitial Zn diffusion. For diffusion into Fe-InP, this low level diffusion produced a region of constant Zn concentration at [Zn] ≈ 3 × 1016 cm−3, due to kick-out of the original Fe species from substitutional sites. We also investigated diffusion out of (Zn, Si) codoped InP epilayers grown on Fe-InP substates. The SIMS profiles were characterised by a sharp decrease in [Zn] at the epilayer-substrate interface; the magnitude of this decrease corresponded to that of the Si donor level in the epilayer. For [Si] ≫ [Zn] in the epilayer no Zn diffusion was observed; Hall measurements indicated that the donor and acceptor species in those samples were electrically active. All these results are consistent with the presence of donor-acceptor interactions in InP, resulting in the formation of ionised donor-acceptor pairs which are immobile, and do not contribute to the diffusion process.