The technology of crystal growth has advanced enormously during the past two decades; among those advances, the development and refinement of molecular beam epitaxy (MBE) has been among the most important. Crystals grown by MBE are more precisely controlled than those grown by any other method, and today form the basis for many of the most advanced device structures in solid-state physics, electronics and optoelectronics.

In addition toits numerous device applications, MBE is also an enormously rich and interesting area ofmaterials science in and of itself. In this paper, we discuss a few examples of some of these materials issues, organized according to whether they involve bulk, thin films, orsurfaces [1].

Molecular beam epitaxy (MBE) is the most powerful crystal growth technique available in terms of the range of structures which can be produced. However, further advances in MBEprocess monitoring and control are required to produce the most demanding structures. Also, process yield, cost, and throughput must be improved for MBE to gain in production worthiness. Because of these issues, there currently is much research activity focusing on the development of advanced in-situ sensors for improved growth parameter monitoring and control. Desorption mass spectrometry (DMS) is an in-situ sensor technique for detecting species leaving the substrate during MBE growth processing. Since in general all surface processes affect desorption rates, and all growth parameters affect surface processes, DMS can be utilized to monitor a variety of MBE growth parameters and phenomena. Theimplementation of simple feedback techniques which link DMS error signals to changes in one or more growth parameters then provides for continuous control of desorption rates. This paper reviews some of the most recent accomplishments in the application of DMS forimproved growth parameter monitoring and control. Additionally, anticipated future directions in the application of DMS to epitaxial growth are discussed.

In the production MBE environment it is important to maintain low densities of oval defects and particle induced defects in epitaxial films that are used for the fabrication ofGaAs ICs. Most often, the grown layers are characterized on a sample basis by use of an optical microscope. The disadvantages of this technique are the time and labor involved.The data obtained is incomplete, dependent on training, and subjective. A preferred method would be to develop an inspection method that characterizes the surface morphology ofall MBE grown GaAs wafers and the resulting defect density. The use of a laser wafer surface scanning system has allowed us to reproducably inspect 100% of wafers. Rapid diagnosis of epitaxial problems has resulted in an improved understanding of how to routinely produce high quality epitaxial films for GaAs IC production. This work will highlight the production benefits derived from employing 100% inspection of MBE grown GaAs wafers and provide 2D maps. The relationship between gallium source operation and defect sizes will be discussed.

We have used scanning room temperature photoluminescence to map GaAs quantum well widthsand AIGaAs barrier compositions over 2-inch and 3-inch diameter epitaxial layers grown by MBE at temperatures ranging from 600 to 700ºC. Analysis of these maps allows a nondestructive quantitative analysis of the GaAs growth rate uniformity from which we can calculate the temperature distribution during growth. We have used this technique to compare the thermal uniformity of various substrate holders designed for use in the Intevac ModGenII MBE system.

We have successfully utilized the temperature dependence of the refractive indices of various substrates (GaAs and InP) in detecting their temperature variations. The detectionwas accomplished by injecting a beam of HeNe laser (1.15 μm) toward a double-sidepolished substrate. An interference pattern developed among the reflected laser beams, and the actual temperature increment/decrement could be deduced from the interference maxima and minima. The method was first calibrated inside a diffusion furnace from room temperature up to 200ºC, and the detection resolution was found to reach as high as ±0.7ºC. The same measurement setup was later used on the temperature calibration of a substrate heater in a molecular beam epitaxy (MBE) growth chamber, and was finally used to detect surface temperature change during Ar+ laser assisted metalorganic MBE (MOMBE) growth. Because a direct contact is unnecessary and only the relative signal intensity is needed, this low-cost method can be applied to any otherexperiments in a fairly straightforward manner.

The Monte Carlo (MC) technique has been frequently used to model semiconductor thin-film epitaxy, especially for the cases of homo-epitaxial growth on Si (100) and GaAs (100) surfaces. In a recent paper, it was shown that excellent qualitative agreement can be obtained between the surface step density evolution as simulated by a simple cubic MC model and the experimentally observed RHEED specular intensity oscillations on stepped GaAs (100) substrates during growth by MBE [1]. The simple cubic model performs well for this case because of the rectangular arrangement of the gallium atoms in the (100) planes of the gallium sublattice. For the case of the (111) surface, however, the gallium atoms in the (111) plane of the gallium sublattice are hexagonally arranged. This system is not amenable to simulation with a simple cubic MC model. In this work, GaAs [111]B epitaxy is modeled using the MC method on a zincblende lattice. To ourknowledge this is the first application of this technique to the growth of a zincblende thin-film in the [11]B direction..The predicted surface step density evolution is compared to experimental RHEED specular intensity oscillations. The relative merits of using on-axis and off-axis substrates are explored.

The temperature dependence of germanium surface segregation during growth by solid source SiGe molecular beam epitaxy was studied by x-ray photoelectron spectroscopy and kinetic Monte Carlo (KMC) modeling. Germanium segregation persisted at temperatures 60ºC below that predicted by a two-state exchange model. KMC simulations, where film growth, surface diffusion, and surface segregation are modeled consistently, successfully describe the low temperature segregation of germanium. Realistic descriptions of MBE must follow the physical rates of the growth, surface diffusion, and surface segregation processes. A specific, step mediated exchange mechanism is also considered and shown to lead to surface segregation. While this model of Ge segregation seems possible, more work is necessary to obtain a consistent set of energy barriers.

A variety of Inl-xGaxAs, Inl-yAlyAs and Inl-x-yGaxAlyAs films have been grown on InP by molecular beam epitaxy. A comprehensive characterization was performed using Raman scattering, photoluminescence (PL), Fourier transform infrared (FTIR) spectroscopy and double crystal X-ray diffraction on these ternary and quaternary heterostructures with different compositions and growth conditions. The lattice matched and mismatched structures are studied. Our analyses show that the interface mismatch exerts an important influence on the optical properties of these heterostructures, and conversely that Raman, PL and FTIR can be used to probe the interface mismatch nondestructively.

In this work we present structural characterization by both Scanning and Transmission Electron Microscopy of InAlAs/InGaAs heterostructures grown on InP substrates. Our attention is devoted to study the two main problems limiting the application of these structures as devices: the presence of defects on the epilayer and the growth habit at theedges of the well. Our results show that a different faceting between the two <110> orthogonal directions develops during the growth and that a high density of defects is observed just at the intersection between the layers grown inside the windows and on the walls. Moreover, the presence of a polycrystalline layer developing over the mask indicates that a selective growth occurs.

We describe the use of molecular beam epitaxy (MBE) for regrowth over a photolithographically defined GaAs shadow mask to fabricate a buried etched void surrounding a vertical-cavity surface-emitting laser (VCSEL). The initial MBE growth consists of a GaAs etch stop layer, followed by an Al0.67Gao.33As spacer layer, and then a GaAs capping layer. After the initial growth, various diameter openings ranging from 2 to10 gtm are photolithographically defined in a photoresist layer, and the top GaAs layer is selectively etched. The exposed LTAlGaAs layer is next selectively etched with the etch time adjusted to undercut the top GaAs layer to create a shadow mask. An MBE regrowthis then performed which consists of alternating layers of GaAs and AlAs. Scanning electron microscopy, surface profiling, and optical microscopy are used to characterize the regrowth.

We have used this shadow masking technique to realize a buried etched void surrounding the optically active part of a VCSEL. A continuous wave room temperature lasing thresholdof 0.47 mA for a 4 [tm device has been achieved, which to our knowledge is the lowest continuous wave room temperature threshold yet reported for a VCSEL.

Advances in gas-source molecular beam epitaxial (GSMBE) growth techniques have allowed the successful fabrication of electronic and photonic devices based on In0.5Ga0.5P and In0.5A10.5P heterostructures lattice matched to GaAs. Basically the interest in In0.5Ga0.5P and In0.5A10.5P derives from their unique material properties as well as their band alignment to GaAs. In this paper, we review the growth, fabrication, and performance of In0.5A10.5P/In0.2Ga0.8As pseudomorphic high electron mobility transistors (HEMT's), InO.5A10.5P/GaAs heterojunction bipolar transistors (HBT's), and In0.5Ga0.5P red light emitting diodes (LED's) grown on Ge/graded GexSil-x/Si substrates. The results provide a solid demonstration of the feasibility of using In0.5Ga0.5P and In0.5A10.5P prepared by GSMBE for manufacturing GaAs-based optoelectronic devices.

lO×16 GaAs/AlGaAs quantum well infrared detector arrays with peak responsivity at 9.3μm have been achieved with front-side normal illumination based on a waveguide of doubly periodic grating coupler. The detector array with a wideband response from 4.2μm to 10.3μm is demonstrated by optimizing the quantum well structures and device processes. A peak detectivity D*λ =2.9×1010 cmHz1/2 /W and high a responsivity Rp=0.37A/W have been obtained at λp=9.3μm and T=77K. The dark current densities of the detectors in the array were smaller than 6.2×10−6 A/cm2 with the detectors biased (Vb=3.2V) for maximum detectivity.

In-situ surface photo-absorption (SPA) method was applied to study the dynamic surface processes and the static surface structures during metal-organic vapor phase epitaxy (MOVPE). Observed spectra in near-ultaviolet and visible regions consist of an anisotropic dielectric response due to surface dimer-bonds as well as an isotropic response probably due to surface back-bonds. Time-resolved spectra showing dynamic changes of surface can be measured by the use of optical multichannel analyzer. The rate of surface decomposition of source molecule was measured as the change of reflectivity at a fixed wavelength. The decomposition of source molecule was characterized quantitatively, and various effects on the decomposition were studied. The style of decomposition and the activation energy depended on the substituent of source molecule and the rate of decomposition was affected by the surface potential and the strain.

Trimethylgallium (TMGa) adsorption on GaAs(100) has been studied by infrared spectroscopy. The adsorbed TMGa exhibits up to five C-H stretching vibrations between 2700 and 3000 cm−1 and five C-H deformations between 1100 and 1500 cm−1. The adsorbate uptake is a maximum at arsenic coverages of –65%. These and other results indicate that TMGa dissociatively adsorbs onto As dimers with methyl transfer to second-layer Ga atoms and to As atoms. Above 333 K, molecular desorption competes with adsorption, causing the saturation coverage to fall with increasing temperature. The desorption kinetics depend on the composition of the GaAs(100) surface. These kinetics suggest that above 773 K, TMGa is more likely to decompose on the c(2×8) than on a more Ga-rich surface.

A concentration wave method for several interpenetrating Bravais sublattices is presented by considering the intralayer and interlayer effective interactions and the difference between the surface layers and the deeper layers in III – V alloys. The ground state ordered structures of ternary III – V semiconductor alloys are deduced and a dynamic model is established.

The band gap of Ga0.51n0.5P is studied as a function of phosphine pressure, B-type substrate misorientation, growth rate, and growth temperature, with emphasis placed on the effect of the phosphine pressure. Over most of the parameter space explored (high temperatures, large substrate misorientations, and low growth rates), the band gap increases with decreasing phosphine. This increase is proposed to be caused by lower phosphorus coverage of the surface, resulting in a different surface structure that doesn't promote ordering. The implications of this effect on the observed variations of band gap with growth temperature, substrate misorientation, and growth rate are discussed. For regions of parameter space in which the ordering appears to be kinetically limited by surface diffusion, the band gap increases slightly with phosphine pressure, consistent with observations that increased group-V pressure decreases the group-III surface diffusion length.

Ordering produced in Ga0.5ln0.5P epitaxial layers grown by OMVPE can be controlled by variations in the substrate misorientation as well as the growth temperature and the growth rate. The ordering produced at a growth temperature of 620°C and a relatively low growth rate of 0.5 μm/hr is found to depend strongly on both the direction and angle of substrate misorientation. Transmission electron microscope images and transmission electron diffraction (TED) patterns as well as electrostatic force microscopy (EFM) and photoluminescence (PL) has been studied for misorientation angles of 0, 3, 6, and 9° from (001) toward the (111)B, (111)A, and [010] directions in the lattice. Misorientation in the (111)B direction (to produce [110] steps) increases ordering for angles of up to approximately 4°. Increasing the misorientation angle in the (111)A direction actually leads to a decrease in the degree of order observed. Misorientation in the [010] direction also decreases the degree of order, although the effect is much less than observed for misorientation in the (111)A direction. The most highly ordered material produced under these growth conditions is for a misorientation angle of 3° in the (111)A direction. Increasing the growth temperature to 720°C produces completely disordered material. This wide variation in ordering behavior has allowed the growth of an order/disorder heterostructure for a substrate misorientation of 3° in the (111)A direction. The heterostructure consists of a Ga0.52In0.48P layer grown at 740°C followed by an ordered layer grown at 620°C. The x-ray diffraction results show that both layers are precisely lattice-matched to the GaAs substrate. TED patterns show that the first layer is completely disordered and the top layer is highly ordered, with only a single variant. EFM images of the order/disorder heterostructure show a pronounced contrast at the interface, attributed to the large difference in the nature of the surface states in the ordered and disordered materials. The 10 K PL spectrum consists of two sharp and distinct peaks at 1.995 and 1.830 eV from the disordered and ordered materials, respectively. The peak separation represents the largest energy difference between ordered and disordered materials reported to date. Such heterostructures may be useful for photonic devices.

The GaInP-GaAs single quantum well structures grown by metal-organic chemical vapor deposition (MOCVD) were investigated by photoreflectance (PR) and photoluminescence (PL) spectroscopies. The built-in electric field in the GaAs close to the top surface was evaluated using the observed Franz-Keldysh oscillations. The fundamental energy gaps and broadening parameters were also determined. The full width at half maximum (FWHM) of the GaInP PL peak is 11 meV

In recent years Metalorganic Chemical Vapor Deposition (MOCVD) becomes a key epitaxial process for a variety of compound semiconductor devices such as: GaAs/AlGaAs lasers, HEMTs, LEDs, photocathodes, solar cells, and MESFETs; InP/InGaAsP long wavelength lasers and detectors; InP/InGaAs quantum wells and detectors, etc. Development of reliable, high throughput equipment is a major task in the implementation of MOCVD into cost-effective manufacturings. We have used Computational Fluid Dynamics (CFD) and Finite Element Analysis (FEA) software to model thermal, structural, and flow processes for the scaling of EMCORE vertical, high speed rotating disk reactor (RDR) to large dimensions (four 4″ wafers located on 12″ wafer carrier). Flow modeling was used to determine basic reactor geometry and the relation between process parameters such as total reactant flow, temperature, pressure, and rotation speed. Thermal and structural analysis was used to produce a uniform substrate temperature, avoid reactor overheating and decrease thermal stress. Flow and temperature distribution predicted by the modeling were found to be well correlated with experimental results.

Compound semiconductors are at the heart of todays advanced digital and optoelectronic devices. As device production levels increase, so too does the need for high throughput deposition systems. The vertical rotating disk reactor (RDR) has been scaled to dimensions allowing metal organic chemical vapor deposition (MOCVD) on multiple substrates located on a 300 mm diameter platter. This symetric large area reactor affords easy access over a wide range of angles for optical monitoring and control of the growth process. The RDR can be numerically modeled in a straightforward manner, and we have derived scaling rules allowing the prediction of optimum process conditions for larger reactor sizes. The material results give excellent agreement with the modeling, demonstrating GaAs/AlAs structures with <±0.9% thickness uniformities on up to 17-50mm or 4-100mm GaAs substrates. Process issues related to reactor scaling are reviewed. With high reactant efficiencies and short cycle times between growths, through the use of a vacuum loadlock, the costs per wafer are found to be dramatically less than in alternative process reactors. The high reactant utilization, in combination with a dedicated and highly efficient exhaust scrubbing system, minimizes the systems environmental impact.