CdZnTe graded buffer layers for HgCdTe/Si integration

To investigate the potential benefits of compositional grading for dislocation control in CdTe/Si growth, Cd_1−xZn_xTe buffer layers with x graded smoothly from 1 to 0 have been deposited on Si (211) surfaces. Growth has been characterized using reflection high-energy electron diffraction (RHEED), x-ray diffraction (XRD), and etch pit density measurements. XRD showed an increase in rocking curve full-width at half-maximum (FWHM) and global lattice tilt with decreasing x values. Tilt was also observed to increase as buffer growth temperature was increased. Final surface dislocation densities did not decrease below 7×10⁶ cm⁻². EPD surface dislocation measurements showed reduced dislocation densities and dislocation clustering along the and lines for CdTe cap layers grown on partially graded Cd_1−xZn_xTe buffer layers with slow compositional grading rates. Samples grown with faster grading rates showed higher final EPD values, with dislocations clustering along the and lines.     

Planar defects and double-domain epitaxy in epitaxial YSi<Subscript>2−x</Subscript> and ErSi<Subscript>2−x</Subscript> thin films on Si substrates

Interfacial reactions of Y and Er thin films on both (111)Si and (001)Si have been studied by transmission electron microscopy (TEM). Epitaxial rare-earth (RE) silicide films were grown on (111)Si. Planar defects, identified to be stacking faults on planes with 1/6 displacement vectors, were formed as a result of the coalescence of epitaxial silicide islands. Double-domain epitaxy was found to form in RE silicides on (001)Si samples resulting from a large lattice mismatch along one direction and symmetry conditions at the silicide/(001)Si interfaces. The orientation relationships are [0001]RESi_2−x// Si, RESi_2−x//(001)Si and [0001]RESi_2−x/ Si, RESi_2−x//(001)Si. The density of staking faults in (111) samples and the domain size in (001) samples were found to decrease and increase with annealing temperature, respectively.     

Identification of prismatic slip bands in 4H SiC boules grown by physical vapor transport

Transmission electron microscopy (TEM) and KOH etching have been used to study the dislocation structure of 4H SiC wafers grown by physical vapor transport. A new type of threading dislocation arrays was observed. Rows of etch pits corresponding to dislocation arrays were observed in vicinity of micropipes, misoriented grains and polytypic inclusions at the periphery of the boules and extended along the directions. Plan view conventional and high resolution TEM showed that the arrays consisted of dislocations threading along the c-axis with Burgers vectors having edge components of the a/3 type. The Burgers vectors were parallel to the corresponding arrays. The dislocation arrays were interpreted as slip bands formed by dislocation glide in the prismatic slip system of hexagonal SiC during post-growth cooling.     

Growth of oriented diamond film on single crystalline 6H-SiC substrates

The bias-enhanced nucleation (BEN) technique in hot-filament chemical vapor deposition (HF-CVD) has been applied to single crystalline 6H-SiC substrates for the deposition of oriented diamond. The results of scanning electron microscopy (SEM) showed that on (000 ) face not only oriented diamond with relationship (111) Dia.//(000 )6H-SiC and 〈110〉Dia.//(11 0)6H-SiC, but also high nucleation density (>10⁹ cm⁻²) have been achieved. In the case of deposition on (0001) face of 6H-SiC under the same experimental conditions, although the nucleation density of diamond was enhanced, however, oriented diamond was not found. Diamond nucleation density is higher on (0001) face than that on (000 ) face. The differences in diamond oriented nucleation and nucleation density on these two faces are attributed to the difference of their specific free surface energy. The experimental results have shown that the 6H-SiC substrate surfaces are etched by the accelerated H-ions during BEN process, and many micro-triangular crystals with the faces of the kind {01 4} are formed on the substrate surface. Diamonds nucleate on the top of the micro-triangular crystals. Micro-Raman spectrum shows a strong feature of diamond crystals at 1334 cm⁻¹.     

Maskless pendeo-epitaxial growth of GaN films

High-resolution x-ray diffraction (XRD) and atomic force microscopy (AFM) of pendeo-epitaxial (PE) GaN films confirmed transmission electron microscopy (TEM) results regarding the reduction in dislocations in the wings. Wing tilt ≤0.15° was due to tensile stresses in the stripes induced by thermal expansion mismatch between the GaN and the SiC substrate. A strong D°X peak at ≈3.466 eV (full-width half-maximum (FWHM) ≤300 μeV) was measured in the wing material. Films grown at 1020°C exhibited similar vertical [0001] and lateral [11 0] growth rates. Increasing the temperature increased the latter due to the higher thermal stability of the GaN(11 0). The (11 0) surface was atomically smooth under all growth conditions with a root mean square (RMS)=0.17 nm.     

The shape of self-assembled InAs islands grown by molecular beam epitaxy

We have determined the shape of InAs quantum dots using reflection high energy electron diffraction. Our results indicate that self-assembled InAs islands possess a pyramidal shape with {136} bounding facets. This shape is characterized by C_2v symmetry and a parallelogram base, which is elongated along the direction. Cross-sectional transmission electron microscopy images taken along the [110] and directions as well as atomic force microscopy images strongly support the {136} shape. Furthermore, polarization-resolved photoluminescence spectra show strong in-plane anisotropy, with emission predominantly polarized along the direction, consistent with the proposed quantum dot shape.     

Effect of off-orientation of seed crystal on silicon carbide (SiC) single-crystal growth on the (11 $$\bar 2$$ 0) surface0) surface

The effect of off-orientation growth has been investigated in terms of stacking fault formation during physical vapor transport (PVT) growth of silicon carbide (SiC) single crystals on the (11 0) seed crystal surface. Occurrence of stacking fault formation is largely dependent on the direction of off-orientation, and basal plane stacking fault density is significantly reduced by growing the crystals on a (11 0) seed crystal off-oriented toward 〈0001〉. The density of the basal plane stacking faults rapidly decreases from 100–150 cm⁻¹ to ∼10 cm⁻¹ as the degree of off-orientation is increased from 0 to 10 deg. The results are interpreted in the framework of microscopic facet formation during PVT growth, and the introduction of off-orientation of seed crystal is assumed to prevent (01 0) and (10 0) microfacet formation on the (11 0) growing surface through modification of the surface growth kinetics and to suppress the stacking fault formation. An erratum to this article is available at .     

A kinetic study of structured surface relief patterning of GaP (\bar 1\bar 1\bar 1)

Orientation dependent etching of photolithographically patterned GaP was investigated using solutions of HCl:CH₃COOH:H₂O₂. The pattern was prepared using standard ultraviolet lithography and was a two-dimensional grid with an 18 μm repeat, consisting of 15 μm squares separated by 3 μm spaces. The mask sides were aligned along the and directions. Under appropriate etching conditions, high quality arrays of pyramids were prepared. These pyramids were defined by , and facets. It was shown that the etching process depended on the degree of solution aging after initial mixing. For a freshly prepared solution, the etching rate showed an inverse dependence on time. For short etching times (below 5 min), an intermediate etching profile was followed, while for long times (greater than 5 min) etching was kinetically controlled. We demonstrated that controlled etching at extremely low rates (0.1–0.5 μm/min) is feasible with this new approach.     

Li diffusion in epitaxial (11 $$\bar 2$$ 0) ZnO thin films

Zinc oxide (ZnO) possesses many interesting properties, such as a wide energy bandgap, large photoconductivity, and high excitonic binding energy. Chemical-vapor-deposition-grown ZnO films generally show n-type conductivity. A compensation doping process is needed to achieve piezoelectric ZnO, which is needed for surface acoustic wave (SAW), bulk acoustic wave, and micro-electromechanical system devices. In this work, a gas-phase diffusion process is developed to achieve piezoelectric (11 0) ZnO films. Comparative x-ray diffraction (XRD) and scanning electron microscopy (SEM) measurements confirmed that high crystal quality and good surface morphology were preserved after diffusion. Photoluminescence (PL) measurements show a broad band emission with a peak wavelength at ∼580 nm, which is associated with Li doping. The SAW, including both Rayleigh-wave and Love-wave modes, is achieved along different directions in piezoelectric (11 0) ZnO films grown on an r-plane sapphire substrate.     

Influence of arsenic on the atomic structure of the Si(112) surface

The surface science techniques of low-energy electron diffraction (LEED), x-ray photoelectron spectroscopy (XPS), and scanning tunneling microscopy (STM) have been used to characterize the clean Si(112) surface and the influence of an As monolayer on the properties and structure of the surface. In agreement with previous studies, the clean surface is found by both LEEd and atomically resolved STM images to be unstable with respect to faceting into other stable planes. Procedures for in-situ deposition of As onto clean Si surfaces were devised and XPS results show that approximately one monolayer of As can be deposited free of any contamination. The As/Si(112) surface is characterized by a sharper LEED pattern than for the clean surface and by STM images characterized by long rows along the direction with a regular width of 1.9 nm. This is consistent with a doubling of the periodicity in the direction of the bulk-terminated unit cell. This implies that As yields a stable but reconstructed Si(112) surface.     

Effects of substrate orientation and surface reconstruction on patterned substrate OMVPE of GaAs

Organometallic vapor phase epitaxial growth of GaAs on 320 nm high mesas was used to study the dependence of lateral growth upon the substrate misorientation from (100) and the mesa wall orientation on the substrate. GaAs (100) substrates were misoriented by 3° toward eight major crystallographic directions, consisting of the four nearest [111] and [110] directions. The mesa sidewalls were oriented either parallel to the 〈011〉 and 〈01 〉 directions or rotated by 45° to be parallel to the 〈001〉 and 〈010〉 directions. GaAs films were grown with TMGa and TBA at T=575°C. The lateral growth rates were up to 25 times higher than the vertical growth rate of 1.3 μm/hour. Optical microscopy and atomic force microscopy (AFM) showed that under the given growth conditions lateral growth off mesa sidewalls is most rapid in the 〈011〉 and/or 〈0 〉 directions and less in the perpendicular 〈01 〉 and 〈0 1〉 directions (lateral growth anisotropy). By raising the temperature to 625°C lateral growth in the 〈01 〉 -〈0 1〉 directions increased while it remained almost constant in the 〈011〉 -〈0 〉 directions. Published results show that the partial pressure of As also affects lateral growth. Differences in the lateral growth rates in the 〈011〉 and its opposite 〈0 〉 directions result from substrate misorientation but not from the orientation of the mesa walls on the substrate. Anisotropic lateral growth rates in different crystallographic directions appear to be caused by both, (1) 1-dimensional Ga diffusion defined by surface reconstruction, and (2) a relatively low energy barrier to atoms flowing over high-to-low terrace steps. A lateral growth model is proposed that describes anisotropic lateral growth at mesa sidewalls in terms of growth conditions and substrate misorientations. The model also explains the difference in the preferential lateral growth directions between MBE and OMVPE.     

Thermal oxidation of polycrystalline and single crystalline aluminum nitride wafers

Two types of aluminum nitride (AlN) samples were oxidized in flowing oxygen between 900°C and 1150°C for up to 6 h—highly (0001) textured polycrystalline AlN wafers and low defect density AlN single crystals. The N-face consistently oxidized at a faster rate than the Al-face. At 900°C and 1000°C after 6 h, the oxide was 15% thicker on the N-face than on the Al-face of polycrystalline AlN. At 1100°C and 1150°C, the oxide was only 5% thicker on the N-face, as the rate-limiting step changed from kinetically-controlled to diffusion-controlled with the oxide thickness. A linear parabolic model was established for the thermal oxidation of polycrystalline AlN on both the Al- and N-face. Transmission electron microscopy (TEM) confirmed the formation of a thicker crystalline oxide film on the N-face than on the Al-face, and established the crystallographic relationship between the oxide film and substrate. The oxidation of high-quality AlN single crystals resulted in a more uniform colored oxide layer compared to polycrystalline AlN. The aluminum oxide layer was crystalline with a rough AlN/oxide interface. The orientation relationship between AlN and Al₂O₃ was (0001) AlN//( ) Al₂O₃ and ( ) AlN//( ) Al₂O₃.     

Atomic hydrogen assisted molecular beam epitaxy on patterned GaAs (311)A substrates: Formation of highly uniform quantum-dot arrays

The idea of combining self-organized growth with growth on patterned substrates to produce new types of nanostructures in a controlled manner is realized in atomic hydrogen assisted molecular beam epitaxy (MBE) on patterned GaAs (311)A substrates. In conventional MBE on patterned substrates mesa stripes along [01 ] develop a fast growing sidewall to form quasi-planar lateral quantum wires having a smooth, convex curved surface profile. In atomic hydrogen assisted MBE, the surface naturally develops quasiperiodic one-dimenional step arrays by step bunching along [ 33], i.e., perpendicular to the wire direction with a lateral periodicity around 40 nm. The step array is maintained over the curved sidewall without displacement. Thus, a dense array of dotlike nanostructures is realized with precise control of the position on the substrate surface. High uniformity of the dot array is revealed in micro-photoluminescence spectroscopy with the emission dominated by one single sharp line.     

Creep behavior of eutectic Sn-Cu lead-free solder alloy

Tensile creep behavior of precipitation-strengthened, tin-based eutectic Sn-0.7Cu alloy was investigated at three temperatures ranging from 303–393 K. The steady-state creep rates cover six orders of magnitude (10⁻³−10⁻⁸ s⁻¹) under the stress range of σ/E=10⁻⁴−10⁻³. The initial microstructure reveals that the intermetallic compound Cu₆Sn₅ is finely dispersed in the matrix of β-Sn. By incorporating a threshold stress, σ _th, into the analysis, the creep data of eutectic Sn-Cu at all temperatures can be fitted by a single straight line with a slope of 7 after normalizing the steady-state creep rate and the effective stress, indicating that the creep rates are controlled by the dislocation-pipe diffusion in the tin matrix. So the steady-state creep rate, , can be expressed as exp , where Q_c is the activation energy for creep, G is the temperature-dependent shear modulus, b is the Burgers vector, R is the universal gas constant, T is the temperature, σ is the applied stress, A is a material-dependent constant, and , in which σ _OB is the Orowan bowing stress, and k_R is the relaxation factor. An erratum to this article is available at .     

Microstructure of GaN grown by lateral confined epitaxy 2. GaN on patterned sapphire

Transmission electron microscopy (TEM), atomic force microscopy (AFM), and photoluminescence (PL) spectroscopy were used in order to study the microstructure and optical properties of GaN films grown by metal-organic chemical vapor deposition (MOCVD) on c-plane sapphire by lateral confined epitaxy (LCE). In this method, the substrate is etched prior to growth to form uniform mesas separated by trenches for laterally restricting growth area. As previously observed for LCE GaN on Si(111), the density of threading dislocations was significantly reduced in the areas close to the edge of mesas due to the lateral propagation of the dislocations. Hence, the overall material quality improves with decreasing mesa size, which is consistent with the observed increase in photoluminescence band edge peak intensity. Electron diffraction indicated ∼1° rotation about the [ ] axis between the mesa and trench material, which was also observed in the image contrast of these two regions with g= . Additionally, LCE samples prepared in [ ] and [ ] cross sections were used for comparing the growth rates in these two perpendicular directions. As theoretically expected, growth in the [ ] direction appears to proceed considerably faster than that in the [% MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9qqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaaGymaiaaig% daceaIYaGbaebacaaIWaaaaa!38D1!\[11\bar 20\]] direction.     

Influence of capping layer thickness on the polarization of photoluminescence of CdSe/ZnSe quantum dots grown by metalorganic chemical vapor phase deposition

The polarization of the photoluminescence (PL) of self-assembled CdSe quantum dots (QDs), grown by metalorganic chemical vapor phase deposition, was measured. From the (001) surface, the PL was found preferentially polarized in the direction, while from the cleaved surface in the [001] direction. The polarization of PL depends strongly on the ZnSe capping layer thickness and the PL energy. With an increase in ZnSe coverage, the intensity ratio was found to increase first, then decrease after the coverage is thicker than a critical value. Moreover, such a critical thickness is smaller for larger QDs (lower PL energies). Possible origins of the PL polarization are discussed. We suggest that besides the quantum confinement effects, the strain field in the QDs also plays an essential role in the observed polarization of PL.     

Laser beam induced current imaging of reactive ion etching induced n-type doping in HgCdTe

The nondestructive optical characterization technique of laser beam induced current (LBIC) has been used to illustrate the effects of reactive ion etching (RIE) of mid-wavelength infrared n-type HgCdTe. RIE may be used as a method of n-p junction formation, as a means of forming n⁺ ohmic contacts to wider bandgap HgCdTe, or for mesa isolation etching of epilayers for HgCdTe detectors and emitters. Along with experimental measurements of the LBIC phenomena, this paper introduces the simulation of LBIC signals using a commercial semiconductor device modeling package. A number of LBIC maps are presented for different wafer processing conditions, with the results being explained using the simulation software. The experimental and calculated results bring to light a number of previously unreported characteristics associated with the LBIC phenomena, including the effect of junction depth, temperature, and grading of the junction region. In addition to the LBIC technique confirming the presence of an n⁺ region after RIE processing, it also provides information regarding the depth of the n⁺ region and lateral extent of the doping.     

Effect of substrate orientation on phase separation in epitaxial GaInAsSb

The effect of substrate misorientation on phase separation in Ga_1−xIn_xAs_ySb_1−y nominally lattice matched to GaSb is reported. The layers were grown at 575°C by organometallic vapor phase epitaxy on vicinal (001) GaSb substrates, miscut or (101). Ga_1−xIn_xAs_ySb_1−y (x ~ 0.1, ~ 0.09) layers, which have 300-K photoluminescence (PL) peak emission at ~2.1 μm, grow stepbunched and exhibit minimal phase separation. The full width at half maximum of 4-K PL spectra is slightly smaller at 7 meV for layers grown on substrates miscut toward compared to 9 meV for layers grown on substrates miscut toward and (101). Ga_1−xIn_xAs_ySb_1−y layers with higher alloy composition (0.16≤x≤0.19, 0.14≤y≤0.17), which have 300-K PL peak emission at ~2.4 μm, have significant phase separation. These layers are characterized by increased lattice constant variations and epitaxial tilt, broad PL spectra with significant band tailing, and strong contrast modulation in transmission electron microscopy. The degree of decomposition depends on substrate miscut direction: Ga_1−xIn_xAs_ySb_1−y layers grown on (001) substrates are more homogeneous than those grown on (001) and (001)2°→(101) substrates. The results are attributed to the smaller adatom diffusion length on substrates miscut toward .     

Zinc diffusion in InAsP/InGaAs heterostructures

A systematic study of the sealed ampoule diffusion of zinc into epitaxially grown InP, In_0.53Ga_0.47As, In_0.70Ga_0.30As, In_0.82Ga_0.18As, and through the InAsP/InGaAs interface is presented. Diffusion depths were measured using cleave-and-stain techniques, electrochemical profiling, and secondary ion mass spectroscopy. The diffusion coefficients, , were derived. For InP, D₀=4.82 × 10⁻²cm²/sec and E_a=1.63 eV and for In_0.53Ga_0.47As, D₀=2.02 × 10⁴cm²/sec and E_a=2.63 eV. Diffusion into the heteroepitaxial structures used in the fabrication of planar PIN photodiodes is dominated by the effects of the InP/InGaAs interface.