Lateral uniformity in HgCdTe layers grown by molecular beam epitaxy

The fabrication of high-quality focal plane arrays from HgCdTe layers grown by molecular beam epitaxy (MBE) requires a high degree of lateral uniformity in material properties such as the alloy composition, doping concentration, and defect density. While it is well known that MBE source flux nonuniformity can lead to radial compositional variation for rotating substrates, we have also found that composition can be affected significantly by lateral variations in substrate temperature during growth. In diagnostic experiments, we systematically varied the substrate temperature during MBE and quantified the dependence of HgCdTe alloy composition on substrate temperature. Based on these results, we developed a methodology to quickly and nondestructively characterize MBE-grown layers using postgrowth spatial mapping of the cutoff wavelength from the Fourier transform infrared (FTIR) transmission at 300 K, and we were able to obtain a quantitative relationship between the measured spatial variations in cutoff and the substrate temperature lateral distribution during growth. We refined this methodology by more directly inferring the substrate temperature distribution from secondary ion mass spectroscopy (SIMS) measurements of the As concentration across a wafer, using the fact that the As incorporation rate in MBE-grown p-type layers is highly sensitive to substrate temperature. Combining this multiple-point SIMS analysis with FTIR spatial mapping, we demonstrate how the relative contributions from flux nonuniformity and temperature variations on the lateral composition uniformity can be separated. This capability to accurately map the lateral variations in the substrate temperature has been valuable in optimizing the mounting and bonding of large substrates for MBE growth, and can also be valuable for other aspects of MBE process development.     

Selective doping of N-type ZnSe layers with chlorine grown by molecular beam epitaxy

N-type ZnSe with electron concentration up to 3 × 10²⁰ cm⁻³ and low resistivity down to 1 × 10⁻⁴ ohm-cm, has been grown using a selective doping technique with chlorine during molecular beam epitaxy. The photoluminescence evaluation shows that the selectively doped ZnSe layers are superior to uniformly doped ones, especially for the case of high-concentration chlorine doping. The in-depth profile of chlorine concentration in a selectively doped sample was measured with secondary-ion mass spectroscopy (SIMS). The SIMS analysis shows only slight diffusion of the incorporated chlorine atoms even in highly doped samples.     

Electrical properties of low-temperature GaAs grown by molecular beam epitaxy and migration enhanced epitaxy

Measurements on low-temperature GaAs epitaxial layers (LT-GaAs) grown by molecular beam epitaxy and migration enhanced epitaxy showed that the excess arsenic incorporated during growth played a crucial role in determining their electrical properties. The electrical transport in LT-GaAs grown by a standard molecular beam epitaxy proceeded mainly via a hopping process, which showed a higher activation energy and onset temperature than those usually observed in lightly doped semiconductors. Using migration enhanced epitaxy to grow LT-GaAs, we were able to substantially reduce the density of As-rich defects and to achieve a good Hall mobility in Be-doped LT-GaAs. The study presented here indicates that, with controlled excess arsenic incorporation during growth, LT-GaAs can vary in a range of conduction properties and thus can be engineered for different device applications.     

The stability of α-Sn grown on CdTe by molecular beam epitaxy

We have examined the effect of the substrate orientation, thickness, growth rate, substrate temperature and inclusion of small amounts of Ge on the transition temperature of α-Sn films grown on CdTe. The transition temperature from α-Sn to β-Sn was determined by optical microscopy to be as high as 132° C. CdTe(ll0) is a somewhat better orientation than CdTe(100), and CdTe(lll)B appears to be totally unacceptable. The transition temperature from α-Sn to β-Sn depends on the film thickness; thinner films have a somewhat higher transition temperature than thicker films. The film quality can be increased by lowering the growth rate and raising the growth temperature to about 75° C. Since the transition from α-Sn to β-Sn starts at defects in the film, improving the film quality by lowering the growth rate and raising the growth temperature raises the transition temperature. Also by adding small amounts of Ge (∼2%) the transition temperature of films grown on CdTe can be significantly increased.     

Growth optimization and doping with Si and Be of high quality GaN on Si(111) by molecular beam epitaxy

GaN layers have been grown by plasma-assisted molecular beam epitaxy on AlN-buffered Si(111) substrates. An initial Al coverage of the Si substrate of aproximately 3 nm lead to the best AlN layers in terms of x-ray diffraction data, with values of full-width at half-maximum down to 10 arcmin. A (2×2) surface reconstruction of the AlN layer can be observed when growing under stoichiometry conditions and for substrate temperatures up to 850°C. Atomic force microscopy reveals that an optimal roughness of 4.6 nm is obtained for AlN layers grown at 850°C. Optimization in the subsequent growth of the GaN determined that a reduced growth rate at the beginning of the growth favors the coalescence of the grains on the surface and improves the optical quality of the film. Following this procedure, an optimum x-ray full-width at half-maximum value of 8.5 arcmin for the GaN layer was obtained. Si-doped GaN layers were grown with doping concentrations up to 1.7×10¹⁹ cm⁻³ and mobilities approximately 100 cm²/V s. Secondary ion mass spectroscopy measurements of Be-doped GaN films indicate that Be is incorporated in the film covering more than two orders of magnitude by increasing the Be-cell temperature. Optical activation energy of Be acceptors between 90 and 100 meV was derived from photoluminescence experiments.     

Selective area growth of GaN using gas source molecular beam epitaxy

Ammonia cracking efficiencies on various surfaces were examined. The following is an ordering of surfaces according to their ammonia cracking efficiencies: GaN (highest), Si₃N₄, SiO₂ (lowest). Selective area growth of GaN was performed over SiO₂ masks deposited on GaN previously grown on sapphire substrates using ammonia-based molecular beam epitaxy. GaN growth on patterned SiO₂/GaN is very selective at a growth temperature of 800°C. Good quality growth occurs in the window region with no deposits on the mask surface when growth is performed at 800°C, whereas some deposits on the SiO₂ masks accumulate when growth is performed at 700°C. The ratio of lateral growth rate to vertical growth rate is ≤1.     

Solid boron and antimony doping of Si and SiGe grown by gas source molecular beam epitaxy

Solid boron and antimony doping of silicon and SiGe grown by molecular beam epitaxy using disilane and germane as sources has been studied. Elemental boron is a well behaved p-type dopant. At effusion cell temperatures of 1700–1750°C, hole carrier concentrations in the 10²⁰ cm⁻³ range have been obtained. Elemental antimony doping shows surface segregation problems. For uniformly doped layers, the as-grown materials do not show n-type conductivity. Electron concentrations in the 10¹⁷ cm⁻³ range were obtained by post-growth conventional and rapid thermal annealing at 900 and 1000°C, respectively. The electron Hall mobility improves with optimum annealing time. Delta doping of buried layers exhibits slightly better incorporation behavior including significant surface riding effects.     

Raman study of defects in a GaAs buffer layer grown by low-temperature molecular beam epitaxy

Raman scattering measurements on high-resistivity layers of GaAs grown by molecular beam epitaxy at low temperature are presented. Several defect-related features are ob-served, including two peaks attributed to quasi-localized vibrational modes of point de-fects, one with a frequency of 223 cm⁻¹ similar to a mode previously observed in elec-tron-irradiated GaAs, and the other with a frequency of 47 cm⁻¹ similar to a mode observed in ion-implanted GaAs. We suggest that these are due to arsenic interstitials and gallium vacancies, respectively. We also observe peaks at 200 and 258 cm⁻¹, which we believe may be due to vibrational modes in small clusters of arsenic. The 223 cm⁻¹ mode is the only defect-related mode still observed after a 10-min annealing treatment at 600° C, although it is significantly broader and has different symmetry from the 223 cm⁻¹ mode in the unannealed material. This indicates that the 223 cm⁻¹ mode in the annealed material is due, at least in part, to a defect other than the arsenic interstitial.     

GaInP and AlInP grown by elemental source molecular beam epitaxy

We report on the use of a new, valved, solid phosphorus cracker source for the growth of phosphides by molecular beam epitaxy. The source avoids the relatively high expense and high level of toxicity associated with the use of phosphine gas and eliminates the problems commonly encountered in using conventional solid phosphorus sources. The source has been used to grow GaInP and AlInP lattice-matched to GaAs substrates. The quality of the materials reported here is comparable to the best materials grown by other techniques. Photoluminescence and Raman scattering measurements indicate that the resulting material has a high degree of disorder on the group III sublattice. The new source is shown to be a reliable and attractive alternative for the growth of these phosphide materials.     

Characteristics and device applications of erbium doped III-V semiconductors grown by molecular beam epitaxy

We have studied the properties of molecular beam epitaxially (MBE)-grown Erdoped III-V semiconductors for optoelectronic applications. Optically excited Er³⁺ in insulating materials exhibits optical emission chiefly around 1.54 μm, in the range of minimum loss in silica fiber. It was thought, therefore, that an electrically pumped Er-doped semiconductor laser would find great applicability in fiber-optic communication systems. Exhaustive photoluminescence (PL) characterization was conducted on several of As-based III-V semiconductors doped with Er, on bulk as well as quantum-well structures. We did not observe any Errelated PL emission at 1.54 μm for any of the materials/structures studied, a phenomenon which renders impractical the realization of an Er-doped III-V semiconductor laser. Deep level transient spectroscopy studies were performed on GaAs and AlGaAs co-doped with Er and Si to investigate the presence of any Er-related deep levels. The lack of band-edge luminescence in the GaAs:Er films led us to perform carrier-lifetime measurements by electro-optic sampling of photoconductive transients generated in these films. We discovered lifetimes in the picosecond regime, tunable by varying the Er concentration in the films. We also found the films to be highly resistive, the resistivity increasing with increasing Er-concentration. Intensive structural characterization (double-crys-tal x-ray and transmission electron microscopy) performed by us on GaAs:Er epilayers indicates the presence of high-density nanometer-sized ErAs precipitates in MBE-grown GaAs:Er. These metallic nanoprecipitates probably form internal Schottky barriers within the GaAs matrix, which give rise to Shockley-Read-Hall recombination centers, thus accounting for both the high resistivities and the ultrashort carrier lifetimes. Optoelectronic devices fabricated included novel tunable (in terms of speed and responsivity) high-speed metal-semiconductor-metal (MSM) photodiodes made with GaAs:Er. Pseudomorphic AlGaAs/ InGaAs modulation doped field effect transistors (MODFETs) (for high-speed MSM-FET monolithically integrated optical photoreceivers) were also fabricated using a GaAs:Er buffer layer which substantially reduced backgating effects in these devices.     

Enhanced electrical properties of nominally undoped Si/SiGe heterostructure nanowires grown by molecular beam epitaxy

Electrical properties of epitaxial single-crystalline Si/SiGe axial heterostructure nanowires (NWs) on Si〈1 1 1〉 substrate were measured by contacting individual NWs with a micro-manipulator inside an scanning electron microscope. The NWs were grown by incorporating compositionally graded Si_1−xGe_x segments of a few nm thicknesses in the Si NWs by molecular beam epitaxy. The I-V characteristics of the Si/SiGe heterostructure NWs showed Ohmic behavior. However, the resistivity of a typical heterostructure NW was found to be significantly low for the carrier concentration extracted from the simulated band diagram. Similarly grown pure Si and Ge NWs showed the same behavior as well, although the I-V curve of a typical Si NW was rectifying in nature instead of Ohmic. It was argued that this enhanced electrical conductivities of the NWs come from the current conduction through their surface states and the Ge or Si/SiGe NWs are more strongly influenced by the surface than the Si ones.     

A study of cracking in GaN grown on silicon by molecular beam epitaxy

It is observed that GaN layers grown on silicon substrates often crack. The crack characteristics in hexagonal GaN films on Si(111) has been characterized using scanning electron microscopy and Nomarski optical microscopy. The effects of growth temperature, layer thickness, and V/III ratios on the cracking have been analyzed. The critical thickness for crack initiation was estimated using a simple theoretical model and is shown to have good agreement with experimental results. Crack-free GaN on Si(111) of thicknesses greater than one micron is possible by using low growth temperatures.     

Doping of molecular beam epitaxy HgCdTe using an arsenic cracker source (As2)

This study investigated the incorporation of arsenic dimers (As₂), delivered from a “cracker” effusion cell. The HgCdTe epilayers were deposited under standard growth conditions. During deposition, arsenic was incorporated using both a standard arsenic effusion cell and a cracker cell. It was found that arsenic concentration rose dramatically as a function of cracker-zone temperature, particularly at temperatures above 600°C. This behavior was consistent with the temperature dependence of the effusion cell’s cracking efficiency, as determined by residual gas analysis. The temporal stability of the arsenic source was excellent. Arsenic concentrations of 2.8×10²⁰ cm⁻³ were achieved at a cracker temperature of 800°C. The arsenic beam-equivalent pressure, estimated from an uncorrected, nude ion-gauge reading, was ∼8×10⁻⁷ mbar.     

Antiphase boundary of GaAs films grown on Si(001) substrates by molecular beam epitaxy

Single and double domain GaAs film growth on double domain Si(001) substrates by molecular beam epitaxy were observed. The domain structure of the film did not succeed to the domain structure of the substrate surface but was decided by a direction of slight misorientation of the substrate. To explain this, it has been proposed that the antiphase boundary (APB) of the film is dominantly non-stoichiometric,i.e., the APB is composed of the same polar {11n} or {nn1} (n = 1, 2, ….) planes of the adjacent domains. Growth simulation based on this model about the APB has explained well the experimental results that a double domain film can grow on a Si(001) surface which is exactly oriented or is misoriented towards a 〈100〉 direction.     

Particulates: A direct origin of oval defects in GaAs layers grown by molecular beam epitaxy

Based on micrographic as well as experimental analyses, we show that particulates which inadvertently adhere to the surface before the wafer is ready for growth are one of the most significant origins for the formation of oval defects on the GaAs layers grown by molecular beam epitaxy. We show that the density of surface particulates is proportional to that of airborne particles surrounding the wafer under preparation, especially during the drying and transferring process. With reduction of airborne particles, we simultaneously reduce the density of oval defects from a few thousand to about 200 cm^-2 for 1-μm thick layers.     

Growth and properties of digitally-alloyed AlGaInP by solid source molecular beam epitaxy

Digital alloying using molecular beam epitaxy (MBE) was investigated to produce AlGaInP quaternary alloys for bandgap engineering useful in 600-nm band optoelectronic device applications. Alternating Ga_0.51In_0.49P/Al_0.51In_0.49P periodic layers ranging from 4.4 monolayers (ML) to 22.4 ML were used to generate 4,000-Å-thick (Al_0.5Ga_0.5)_0.51In_0.49P quaternary materials to understand material properties as a function of constituent superlattice layer thickness. High-resolution x-ray diffraction (XRD) analysis exhibited fine satellite peaks for all the samples confirming that digitally-alloyed (Al_0.5Ga_0.5)_0.51In_0.49P preserved high structural quality consistent with cross-sectional transmission electron microscopy (X-TEM) images. Low-temperature photoluminescence (PL) measurements showing a wide span of luminescence energies ～ 170 meV can be obtained from a set of identical composition digitally-alloyed (Al_0.5Ga_0.5)_0.51In_0.49P with different superlattice periods, indicating the bandgap tunability of this approach and its viability for III-P optoelectronic devices grown by MBE.     

Influence of alloy composition,substrate temperature,and doping concentration on electrical properties of Si-doped n-Alx Ga1−x As grown by molecular beam epitaxy

Capacitance and Hall effect measurements in the temperature range 10-300 K were performed to evaluate the deep and shallow level characteristics of Si-doped n-Al_xGa_-xAs layers with 0 × 0.4 grown by molecular beam epitaxy. For alloy compositions × 0.3 the overall trap concentration was found to be less than 10⁻² of the carrier concentration. In this composition range the transport properties of the ternary alloy are comparable to those of n-GaAs:Si except for lower electron mobibities due to alloy scattering. With higher Al content one dominant electron trap determines the overall electrical properties of the material, and in n-Al_0.35Ga_0.65As:Si the deep trap concentration is already of the order of the free-carrier concentration or even higher. For the composition × = 0.35 ± 0.02 the influence of growth temperature and of Si dopant flux intensity on the deep trap concentration, on shallow and deep level activation energy, and on carrier freeze-out behaviour was studied and analyzed in detail. Our admittance measurements clearly revealed that the previously assumed deepening of the shallow level in n-Al_x Ga_1-x As of alloy composition close to the direct-indirect cross-over point does actuallynot exist. In this composition range an increase of the Si dopant flux leads to a reduction of the thermal activation energy for electron emission from shallow levels due to a lowering of the emission barrier by the electric field of the impurities. The increasing doping flux also enhances the concentration of the dominant electron trap strongly, thus indicating a participation of the dopant atoms in the formation of deep donor-type (D,X) centers. These results are in excellent agreement with the model first proposed by Lang et al. for interpretation of deep electron traps in n-Al_x Ga_1-x grown by liquid phase epitaxy.     

Origin of void defects in Hg1−xCdxTe grown by molecular beam epitaxy

Characterization of defects in Hg_1−xCd_xTe compound semiconductor is essential to reduce intrinsic and the growth-induced extended defects which adversely affect the performance of devices fabricated in this material system. It is shown here that particulates at the substrate surface act as sites where void defects nucleate during Hg_1−xCd_xTe epitaxial growth by molecular beam epitaxy. In this study, we have investigated the effect of substrate surface preparation on formation of void defects and established a one-to-one correlation. A wafer cleaning procedure was developed to reduce the density of such defects to values below 200 cm⁻². Focal plane arrays fabricated on low void density materials grown using this new substrate etching and cleaning procedure were found to have pixel operability above 98.0%.     

Unintentional incorporation of B, As, and O impurities in GaN grown by molecular beam epitaxy

We have investigated systematically the effects of growth parameters upon the unintentional incorporation of B, As, and O impurities in GaN grown by molecular beam epitaxy with an RF-plasma activated nitrogen source. The prepared samples were analyzed using secondary ion mass spectrometry to determine the absolute concentration of the impurities. The boron background concentration in the unintentionally doped GaN was found to strongly correlate with the nitrogen plasma power used during the growth, indicating a decomposition of the pBN crucible in the plasma source. Due to previous GaAs growth in the same chamber, a considerably large amount of As contamination (≈3×10¹⁸ at/cm³) was also observed in the grown layer. The presence of Al in GaN is found to facilitate the incorporation of oxygen impurities in the layer. We determined an empirical formula, C_o ^t/C_o ^b 3.8×(C_Al/C_Al)^0.27, representing the correlation between O concentration and Al mole fraction (%) in the small range of Al content, 0.03≈1%, in the layer. The residual oxygen level was substantially reduced from 3.4×10¹⁹ to mid-10¹⁸ at/cm³ in the GaN layer when the buffer layer structure was changed from low temperature grown GaN single buffer to GaN/AlN double buffer layer. We ascribe this significantly lowered oxygen impurity level to improved crystalline quality of the layer due to the double buffer layer structure.     

Crystal quality of InN thin films grown on ZnO substrate by radio-frequency molecular beam epitaxy

The growth of InN on {0001} ZnO substrates by radio-frequency, plasma-assisted, molecular-beam epitaxy (RF-MBE) has been experimentally investigated. The reflection high-energy electron diffraction (RHEED) pattern quickly recovered to a 1×1 streak pattern as the InN growth was started on nitridated ZnO substrates, whereas the RHEED pattern of the ZnO substrate was spotty because of plasma damage induced by nitridation. The full width at half maximum (FWHM) of the (0002) InN rocking curve was estimated to be around 150 arcsec from x-ray diffraction (XRD). Furthermore, we observed a remarkable feature from our experiments; namely, the crystal quality of InN does not seem to depend on the surface polarity of the ZnO substrate, while it is well known that InN growth on GaN has strong polarity dependence. To investigate this tendency, we have also investigated the surface stability of adatoms, In and N, on Zn- and O-face ZnO surfaces using a first-principles technique. From the theoretical study, N adsorption is more stable on ZnO surfaces of both polarities compared with In adsorption. Accordingly, the preferential initiation by N adatoms onto both ZnO surfaces can explain the unique style of InN growth on ZnO substrates.