Formation and control of defects during molecular beam epitaxial growth of HgCdTe

Void defects were demonstrated to form away from the substrate-epifilm interface during the molecular beam epitaxial growth of mercury cadmium telluride on cadmium zinc telluride substrates. These were smaller in size compared to voids which nucleated at the substrate-epifilm interface, which were also observed. Observations of void nucleation away from the substrate-epifilm interface were related to the respective growth regimes active at the time of the void nucleation. Once nucleated, voids replicated all the way to the surface even if the flux ratios were modified to prevent additional nucleation of voids. For a significant number of films, void defects were observed co-located with hillocks. These voids were usually smaller than 1 μm and appeared almost indistinguishable from unaccompanied simple voids. However, these void-hillock complexes displayed a nest of dislocation etch pits around these defects upon dislocation etching, whereas unaccompanied voids did not. The nests could extend as much as 25 μm from the individual void-hillock complex. The density of dislocations within the nest exceeded 5×10⁶ cm⁻², whereas the dislocation density outside of the nest could decrease to <2×10⁵ cm⁻². The void-hillock complexes formed due to fluctuations in growth parameters. Elimination of these fluctuations drastically decreased the concentrations of these defects.     

Hopping conduction and its photoquenching in molecular beam epitaxial GaAs grown at low temperatures

As the growth temperature of molecular beam epitaxial GaAs is increased from 250 to 400°C, the dominant conduction changes from hopping conduction to band conduction with a donor activation energy of 0.65 eV. A 300°C grown layer is especially interesting because each conduction mechanism is dominant in a particular temperature range, hopping below 300K and band conduction above. Below 140K, the hopping conduction is greatly diminished (quenched) by irradiation with either infrared (hv≤1.12 eV) or 1.46 eV light, but then recovers above 140K with exactly the same thermal kinetics as are found for the famous EL2. Thus, the 0.65 eV donor, which is responsible for both the hopping and band conduction, is very similar to EL2, but not identical because of the different activation energy (0.65 eV vs 0.75 eV for EL2).     

Electrical properties of molecular beam epitaxial GaAs grown at 300–450°C

We use the Hall effect and a new charge-transfer technique to study molecular beam epitaxial GaAs grown at the low substrate temperatures of 300–450°C. Layers grown from 350–450°C are semi-insulating (resistivity greater than 10⁷ Ω-cm), as grown, because of an As_Ga-related donor (not EL2) at E_C-0.65 eV. The donor concentrations are about 2×10¹⁸ cm⁻³ and 2×10¹⁷ cm⁻³ at growth temperatures of 300 and 400°C, respectively, and acceptor concentrations are about an order of magnitude lower. Relatively high mobilities (∼5000 cm²/V s) along with the high resistivities make this material potentially useful for certain device applications.     

Infrared studies of be-doped GaAs grown by molecular beam epitaxy at low temperatures

Samples of molecular beam epitaxial GaAs grown at low temperatures doped with Be defects are studied as a function of growth temperature (T_G)-by measuring their localized vibrational modes at 77K using BOMEM Fourier transform infrared spectrometer. Localized vibrational modes of⁹Be_Ga in samples grown at T_G>350°C have been identified at 482 cm⁻¹. Secondary ion mass spectroscopy measurements show that the densities of Be defects remain approximately constant as T_G is lowered, however, additional structure in the⁹Be_Ga localized vibrational mode is observed. Calculations based on Green's function theory suggest that the additional structure in Be-doped LT GaAs can best be explained in terms of a complex center [⁹Be_Ga-As_Ga] involving an intrinsic defect.     

Effects of heat treatment on the 0.8 eV photoluminescence emission in GaAs grown by molecular beam epitaxy at low temperatures

We report 0.8 eV photoluminescence (PL) emission of GaAs grown at low temperatures between 325 and 400°C by molecular beam epitaxy. Effects of heat treatments of the 0.8 eV emission are compared with those of the 1.467 eV sharp bound exciton lines. This allows us to attribute the 0.8 eV emisson to the As-V_Ga center. We discuss the assigning of the As_i-V_Ga center to the well-known EL6. The PL intensity variation of 0.68 eV EL2 and 0.8 eV As_i-V_Ga seen in substrate materials is explained in terms of dislocation−mediated As_i-V_Ga transformation to EL2 whereas the PL intensity variation of 0.8 eV As_i-V_Ga for molecular beam epitaxy layers can be attributed to the growth condition.     

Thick crack-free CaF2 epitaxial layer on GaAs (100) substrate by molecular beam epitaxy

The crystallographic orientation of low temperature (LT) grown CaF₂ on GaAs (100) substrates is investigated. LT epitaxial (100) CaF₂ layers are obtained on a thin (100) oriented CaF₂ template at growth temperatures down to room temperature. This makes it possible to grow crack-free CaF₂ (100) using a multiple-temperature-growth scheme at any desired temperature. The resulting CaF₂ layers, with thickness up to 680 nm, can withstand temperature cycling from RT to 650°C without cracking. Based on these results, a four pair Ga_0.5Al_0.5As/CaF₂ quarter-wavelength Bragg reflector was fabricated with center wavelength at 880 nm. The reflector, with a total CaF₂ thickness of 615 nm, shows broadband high reflectivity with a crack-free surface. This crack-free surface can then be overgrown with further device layers.     

InAs/GaSb超晶格中波焦平面材料的分子束外延技术

报道了InAs/GaSb超晶格中波材料的分子束外廷生长技术研究.通过改变GaSb衬底上分子束外延InAs/GaSb超晶格材料的衬底温度,以及界面的优化等,改善超晶格材料的表面形貌和晶格失配,获得了晶格失配△a/a=1.5×10-4,原子级平整表面的InAs/GaSb超晶格材料,材料77 K截止波长为4.87 μm.     

Mobility of modulation doped AlGaAs/low-temperature MBE-grown GaAs heterostructures

A study of the mobility of a novel modulation doped heterostructure in which the channel region is made of low-temperature molecular beam epitaxially grown GaAs (LT-GaAs) and all other layers are grown at normal temperatures is presented for the first time. The resistivity of the as-grown samples(in- situ annealed) is very high, as is that of single layers of bulk LT-GaAs. However, in the presence of light, the resistivity of the LT-GaAs modulation-doped field effect transistor (MODFET) is significantly lower, facilitating reliable Hall measurements. We speculate that the observed decrease in resistivity of the LT-GaAs MODFET is due to the formation of a two-dimensional electron gas (2DEG) at the heterointerface under illumination. A number of samples grown under different growth conditions were investigated. Mobilities for these samples were found to be in the range of 250 to 750 cm²Vs at 300K and ∼3000 to 5500 cm²Vs at 77K. A first-order computer simulation was implemented to calculate the mobility of the 2DEG using the relaxation-time approximation to solve the Boltzmann equation, taking into account different scattering mechanisms. Scattering by the arsenic clusters and by ionized impurities in the LT-GaAs MODFET channel are found to be the two dominant mechanisms limiting the mobility of the LT-GaAs MODFET samples. Experimental values are in good agreement with theoretical results.     

Transmission electron microscopy studies of defects in HgCdTe device structures grown by molecular beam epitaxy

Transmission electron microscopy (TEM) was used to evaluate the microstructure of molecular beam epitaxy (MBE) grown (211)B oriented HgCdTe films. TEM analysis of in-situ doped p-on-n and n-p-n device structures will be presented. Under fully optimized growth conditions the substrate-epilayer interface is free of threading dislocations and twins, and a high degree of structural integrity is retained throughout the entire device structure. However, under non-optimal growth conditions that employ high Hg/Te flux ratios, twins can be generated in the p-type layer of p-on-n device structure, resulting in roughness and facetting of the film surface. We propose a mechanism for twin formation that is associated with surface facetting. TEM evaluation of voids, threading dislocations and Te-precipitates in HgCdTe films are also discussed.     

Two-dimensional molecular beam epitaxy of {001} CdTe on Cd and Zn terminated {001} GaAs

Amorphous layers of CdTe deposited on Cd or Zn terminated GaAs {001} surfaces can be recrystallized above ∼200°C. Subsequent molecular beam epitaxy of CdTe proceeds in a two-dimensional mode and leads to layers which are specular and single domain {0011}. Threading dislocation density in these layers was 1–2 x 10⁵ cm⁻². Values of full width at half maximum for x-ray rocking curves were as low as 80 arc-s.     

Uniform low defect density molecular beam epitaxial HgCdTe

This paper describes recent advances in MBE HgCdTe technology. A new 3 inch production molecular beam epitaxy (MBE) system, Riber Model 32P, was installed at Rockwell in 1994. The growth technology developed over the years at Rockwell using the Riber 2300 R&D system was transferred to the 32P system in less than six months. This short period of technology transfer attests to our understanding of the MBE HgCdTe growth dynamics and the key growth parameters. Device quality material is being grown routinely in this new system. Further advances have been made to achieve better growth control. One of the biggest challenges in the growth of MBE HgCdTe is the day-to-day control of the substrate surface temperature at nucleation and during growth. This paper describes techniques that have led to growth temperature reproducibility within + - 1°C, and a variation in temperature during substrate rotation within 0.5°C. The rotation of the substrate during growth has improved the uniformity of the grown layers. The measured uniformity data on composition for a typical 3 cm × 3 cm MBE HgCdTe/CdZnTe shows the average and standard deviation values of 0.229 and 0.0006, respectively. Similarly, the average and standard deviation for the layer thickness are 7.5 and 0.06 μm, respectively. P-on-n LWIR test structure photodiodes fabricated using material grown by the new system and using rotation during growth have resulted in high-performance (R₀)A, quantum efficiency) devices at 77 and 40K. In addition, 128 × 28 focal plane arrays with excellent performance and operability have been demonstrated.     

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.     

Incorporation of Sn on GaAs (111)A substrates by molecular beam epitaty

Behavior of Sn as donor species in the MBE growth of GaAs on (111)A substrates has been investigated by varying the growth temperature from 460 to 620°C, As₄:Ga flux ratio from 4 to 25, and Sn concentration from 10¹⁶ to 10²⁰ atoms cm^-3. Secondary ion mass microscopy measurements show that Sn does not surface segregate on (111)A substrates under this growth condition, in contrast to that on (001) substrates. Sn is uniformly incorporated throughout the bulk of the grown layer for all samples, apart from the most highly doped ones. To increase the Sn carrier concentration on the (111)A substrates, the measured carrier concentration shows that doping should be carried out at a low growth temperature and/or high As₄:Ga flux ratio.     

Characterization of molecular beam epitaxially grown HgCdTe epilayers by mid-infrared interband magneto-absorption

Interband magneto-absorption is used to characterize molecular beam epitaxially (MBE) grown HgCdTe epilayers. Both the bandgap and the Moss-Burstein shift in n-doped layers are determined from the experiments. A heterostructure sample consisting of four layers with different compositions is also analyzed. Due to the good experimental sensitivity all four bandgaps are determined, in contrast to optical transmission analysis without a magnetic field where only the lowest gap is readily visible. The interband magneto-absorption signal strongly depends on the electron mobility. This has been used as an aid to optimizing the MBE growth conditions of HgCdTe layers on different substrate orientations.     

Real-time monitoring of the surface stoichiometry during molecular beam epitaxy of cubic GaN on (001) GaAs by RHEED

We study the plasma-assisted molecular beam epitaxy of cubic GaN on GaAs(OOl) substrates by means ofin-situ reflection high-energy electron diffraction. The epilayers are characterized by x-ray diffraction, photoluminescence, and Hall measurements, and it is found that the overall best films are grown under a N/Ga ratio close to one. For anin-situ determination of the N/Ga ratio, the growth kinetics is studied via surface reconstruction transitions. The effective N flux giving rise to growth is measured using the transient behavior of the half-order diffraction streak intensity for various plasma operating conditions.     

Growth of II-IV-V<Subscript>2</Subscript> chalcopyrite nitrides by molecular beam epitaxy

Synthesis of crystalline MgGeN₂ thin solid films is achieved using the technique of molecular beam epitaxy (MBE). The details of the epitaxial process are described. The microstructures of these films are investigated by both x-ray diffraction (XRD) and cross-sectional transmission electron microscopy (XTEM). Comparison of the lattice structure with powder diffraction standards suggests the lattice structure may be orthorhombic with a high degree of texture. Morphology is evaluated by atomic force microscopy, and a periodic pattern of growth mounds is observed. A formalism for dynamical roughening is applied to quantify the mounded surface features. Mounds are found to have an average spacing of 235 nm, and the surface exhibits a saturation value of 22 nm for the root mean correlated height difference. Diffusion bias is discussed as a mechanism for the formation of surface mounds.     

Hall and drift mobilities in molecular beam epitaxial grown GaAs

A series of nominally undoped and Si-doped GaAs samples have been grown by molecular beam epitaxy (MBE) with Hall concentrations ranging from 10¹⁵ to 10¹⁹ cm⁻³ and mobilities measured at 77 and 300K by Hall-van der Pauw methods. Drift mobilities were calculated using the variational principle method and Hall scattering factors obtained from a relaxation-time approximation to permit cross-correlation of experimental data with drift or Hall mobilities and actual or Hall electron concentrations. At 77K, both high purity and heavily doped samples are well represented by either drift or Hall values since piezoelectric acoustic phonon scattering and strongly screened ionized impurity scattering hold the Hall factor close to unity in the respective regimes. Between n≊10¹⁵ and 10¹⁷ cm⁻³, where lightly screened ionized impority scattering predominates, Hall mobility overestimates drift mobility by up to 50 percent and Hall concentration similarly underestimates n. At 300K, polar optical phonons limit mobility and a Hall factor up to 1.4 is found in the lowest doped material, falling close to unity above about 10¹⁶ cm⁻³. Our calculation also agrees remarkably well with the Hall mobility of the highest purity MBE grown sample reported to date.     

Strain effects in CdTe (111) epitaxial layers grown on GaAs (100) substrates by molecular beam epitaxy

Spectroscopic ellipsometry and photoreflectance measurements on CdTe/GaAs strained heterostructures grown by moleculclr beam epitaxy were carried out to investigate the effect of the strain and the dependence of the lattice parameter on the CdTe epitaxial layer thicknesses. Compressive strains exist in CdTe layers thinner than 2 μm. As the strain increases, the value of the critical-point energy shift increases linearly. These results indicate that the strains in the CdTe layers grown on GaAs substrates are strongly dependent on the CdTe layer thickness. Author to whom all correspondence should be addressed.     

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%.     

Characterization of low range GaAs

We present the results of a study of GaAs material grown at substrate temperatures below 250°C (low range GaAs) by molecular beam epitaxy. This material is amorphous and highly resistive and can be converted to single crystal through annealing process. The crystallization process is investigated by transmission electron microscopy, reflection high-energy electron diffraction, and double-crystal x-ray diffraction techniques.