Organometallic Vapor Phase Epitaxial Growth Research Articles

In Situ Smoothing of Facets on Spalled GaAs(100) Substrates during OMVPE Growth of III-V Epilayers, Solar Cells, and Other Devices: The Impact of Surface Impurities/Dopants.

One possible pathway toward reducing the cost of III-V solar cells is to remove them from their growth substrate by spalling fracture, and then reuse the substrate for the growth of multiple cells. Here we consider the growth of III-V cells on spalled GaAs(100) substrates, which typically have faceted surfaces after spalling. To facilitate the growth of high-quality cells, these faceted surfaces should be smoothed prior to cell growth. In this study, we show that these surfaces can be smoothed during organometallic vapor-phase epitaxy growth, but the choice of epilayer material and modification of the various surfaces by impurities/dopants greatly impacts whether or not the surface becomes smooth, and how rapidly the smoothing occurs. Representative examples are presented along with a discussion of the underlying growth processes. Although this work was motivated by solar cell growth, the methods are generally applicable to the growth of any III-V device on a nonplanar substrate.

Characterization of defects and strain in the (AlxGa(1−x))0.5In0.5P/ GaAs system by synchrotron X-ray topography

Abstract Synchrotron white beam X-ray topography and monochromatic beam topography have been applied to study GaAs substrates and (AlxGa(1−x))0.5In0.5P epitaxial layers grown on them that are being developed for LEDs. The GaAs substrates are 15° offcut toward [1 1 0] and mainly consist of dislocations in the {1 1 1} slip planes and low angle grain boundaries. The nominally lattice matched (AlxGa(1−x))0.5In0.5P layers contain defects such as threading dislocations replicated from the substrates, inclusions and misfit dislocations generated during organometallic vapor phase epitaxy (OMVPE) growth. The existence of misfit dislocations (MDs) indicates that strain relaxation has taken place to accommodate the in-plane lattice mismatch between epitaxial layer and substrate. The driving force for the glide of threading dislocations and relaxation efficiency of dislocations with different Burgers vectors and glide planes has been analyzed in the wafers with 15° offcut to predict the Burgers vectors and line directions of dislocations that dominate strain relaxation. Burgers vectors have been determined using ray tracing simulations. Reciprocal space maps (RSMs) recorded from the wafers to measure the strain and tilt of the epilayers show both in-plane and out-of-plane lattice mismatch as well as tilt between epilayer and substrate. A modified Nagai′s model has been developed to analyze and account for in-plane and out-of-plane tilt.

Characteristics of OMVPE grown GaAsBi QW lasers and impact of post-growth thermal annealing

Laser diodes employing a strain-compensated GaAs1−xBix/GaAs1−yPy single quantum well (SQW) active region were grown by organometallic vapor phase epitaxy (OMVPE). High resolution x-ray diffraction, room temperature photoluminescence, and real-time optical reflectance measurements during the OMVPE growth were used to find the optimum process window for the growth of the active region material. Systematic post-growth in situ thermal anneals of various lengths were carried out in order to investigate the impacts of thermal annealing on the laser device performance characteristics. While the lowest threshold current density was achieved after the thermal annealing for 30 min at 630 °C, a gradual decrease in the external differential quantum efficiency was observed as the annealing time increases. It was observed that the temperature sensitivities of the threshold current density increase while those of lasing wavelength and slope efficiency remain nearly constant with increasing annealing time. Z-contrast scanning transmission electron microscopic) analysis revealed inhomogeneous Bi distribution within the QW active region.

Development of High-Bandgap AlGaInP Solar Cells Grown by Organometallic Vapor-Phase Epitaxy

AlGaInP solar cells with bandgaps between 1.9 and 2.2 eV are investigated for use in next-generation multijunction photovoltaic devices. This quaternary alloy is of great importance to the development of III–V solar cells with five or more junctions and for cells optimized for operation at elevated temperatures because of the high bandgaps required in these designs. In this work, we explore the conditions for the organometallic vapor-phase epitaxy growth of AlGaInP and study their effects on cell performance. Initial efforts focused on developing ∼2.0-eV AlGaInP solar cells with a nominal aluminum composition of 12%. Under the direct spectrum at 1000 W/m2 (AM1.5D), the best of these samples had an open-circuit voltage of 1.59 V, a bandgap-voltage offset of 440 mV, a fill factor of 88.0%, and an efficiency of 14.8%. We then varied the aluminum composition of the alloy from 0% to 24% and were able to tune the bandgap of the AlGaInP layers from ∼1.9 to ∼2.2 eV. While the samples with a higher aluminum composition exhibited a reduced quantum efficiency and increased bandgap-voltage offset, the bandgap-voltage offset remained at 500 mV or less, up to a bandgap of ∼2.1 eV.

Lattice-Matched and Strain-Compensated Materials for Mid-Wave and Long-Wave Infrared Quantum Cascade Lasers

This work reports the growth and characterization of lattice-matched (LM) and strain-compensated (SC) AlInAs/GaInAs/InP heterostructures for quantum cascade lasers (QCLs) emitting in the mid-wave infrared (MWIR) and long-wave infrared (LWIR) wavelength regions. Organometallic vapor phase epitaxy growth conditions such as temperature, growth rate, and V/III ratio were varied to establish a step-flow growth mode. Multiple-quantum-well structures with either (LM) or with SC compressive GaInAs quantum wells and tensile AlInAs barriers were grown to optimize QCL structure growth. Atomic force microscopy and high-resolution x-ray diffraction data indicate excellent structural quality materials for QCLs. LM LWIR QCLs at 9.6 μm have pulsed peak power as high as 2.5 W/facet and pulsed wall-plug efficiency of 5.4%, while SC MWIR QCLs at 4.8 µm have peak power of 1.7 W and 9.7% pulsed wall-plug efficiency. SC MWIR QCLs exhibit very low threshold current density of 0.8 kA/cm2.

Effects of dimethylhydrazine on Zn, C, and H doping of GaP

Surfactants are beginning to be used during vapor-phase growth to provide control of the materials characteristics of epitaxial layers. This paper reports the effects of utilizing N, from pyrolysis of dimethylhydrazine (DMHy), as a surfactant during the organometallic vapor-phase epitaxial growth of GaP. Nitrogen is isoelectronic with the group V host element; thus, it generates no free carriers. Additionally, due to its small size, N is strongly rejected from the solid, so little incorporation is expected. This is confirmed by the determination of the very low N distribution coefficient (3.2×10 −5). Using secondary ion mass spectroscopy analysis, we observe an increase in the distribution coefficient of the acceptor Zn when DMHy is added during growth, resulting in an increase in the doping efficiency by nearly an order of magnitude. Furthermore, the Zn doping efficiency increases linearly with the amount of DMHy added during growth over the range of partial pressures from 2.83×10 −4 to 2.20×10 −3 atm. The increase of doping for Zn is believed to be mainly a N surfactant effect. The addition of DMHy also leads to increases in both H and C incorporation. The concentrations of both are linear functions of DMHy concentration. Both H and C incorporation most likely arise mainly from pyrolysis of the DMHy. We propose a simple mechanism for the surfactant effect due to an increase in the Zn incorporation coefficients at the step edge induced by the presence of N (and probably H) on the surface.

Effects of low surfactant Sb coverage on Zn and C incorporation in GaP

The use of surfactants during the vapor phase growth of III-V materials to control fundamental characteristics of epitaxial layers is becoming increasingly important. We have investigated the remarkable effects of Sb, from triethylantimony (TESb) pyrolysis, on the Zn doping during the organometallic vapor phase epitaxial growth (OMVPE) of GaP. Antimony is isoelectronic with the P host; therefore it is not a dopant in this material. It is also much larger than P so little incorporation occurs. We used secondary ion mass spectroscopy (SIMS) to investigate in detail the effects of TESb flow rate (Sb surface coverage) on the incorporation of the dopant Zn, as well as the background impurities C and H in GaP. The doping efficiency of Zn increased by as much as a factor of 2 when Sb was added during growth. Importantly, the observed effect was steady throughout the entire range of Sb levels from an Sb/III ratio of 0.01–0.05. Previous studies indicate that this would yield Sb surface coverages of 0.3 (for Sb/III=0.01) to 0.65 (for Sb/III=0.05). Thus, the Sb surface coverage is not saturated. Other results indicate that Sb coverage may be higher at the step edge, which suggests that the surfactant effect on Zn doping occurs at the step edge. Additionally, Sb caused an increase in the hydrogen concentration and a reduction in the carbon contamination. We propose a simple mechanism for the surfactant effect on Zn doping due to an increase of Sb coverage near the step edge.

Effects of surfactants Sb and Bi on the incorporation of zinc and carbon in III/V materials grown by organometallic vapor-phase epitaxy

The incorporation of both dopants and background impurities during the organometallic vapor phase epitaxial (OMVPE) growth of GaAs, GaInP, and GaP has been significantly altered by the use of the surfactants Sb and Bi. Sb and Bi are isoelectronic with the group V host elements, and so produce no independent doping. This paper demonstrates that the incorporation of Zn can be increased by an order of magnitude in GaP to a value of approximately 1020cm−3, the highest value reported to date. Additionally, these same surfactants lead to significant decreases in carbon contamination during growth. At high growth temperatures the reduction can be as large as 100× in GaP. The surfactants also result in marked decreases in autodoping due to S and Si from the substrate. A marker that may help identify the mechanism for these effects is H incorporation. Both Sb and Bi surfactants give rise to increased concentrations of H in the GaP layers. Similar effects are observed in GaInP. However, in GaAs, no H is detected in the layers. One possible explanation for these phenomena is that Sb or Bi increases the surface H concentration. The increased H would have two possible effects on the incorporation of dopants and impurities. (1) Passivation of the Zn acceptor by formation of a neutral Zn-H complex would lead to increased incorporation for thermodynamic reasons. (2) Allowing increased desorption of C, S, and Si from the surface by increased formation of volatile hydrides leads to decreased incorporation levels. These results suggest a simple and effective method of controlling the incorporation of dopant and impurity atoms by adding a minute amount of surfactant during OMVPE growth.

Zn enhancement during surfactant-mediated growth of GaInP and GaP

The addition of surfactants during organometallic vapor phase epitaxy (OMVPE) growth is a novel approach to controlling the surface processes and materials properties of the resultant epitaxial layers. The present work elucidates the effects of the surfactants Sb and Bi on dopant incorporation in GaP and GaInP. The effects of Sb and Bi surfactants are dramatic. The surfactants significantly increase the incorporation of the acceptor Zn in both GaP and GaInP. Also, increased H concentrations are observed. A simple mechanism is proposed to explain both the increase in Zn and H. These results suggest a simple and effective method for controlling the incorporation of dopants by adding a minute amount of surfactant during OMVPE growth.

Ab initio molecular orbital study on atmosphere-sensitive characteristics of the alternate supply OMVPE growth of ZnSe

An insight into surface chemical reactions responsible for the organometallic vapor phase epitaxial (OMVPE) growth of ZnSe under the alternate supply of diethylzinc (DEZn) and dimethylselenium (DMSe) was provided by theoretical interpretation of our previous experimental results. Based on a quantum chemical study, atmosphere-sensitive characteristics of ZnSe growth were attributed to differences in the pathways and the corresponding adiabatic potential barriers between diethylzinc chemisorption onto hydrogen- and methyl-terminated Se-stabilized ZnSe surfaces.

Effects of Br and Cl on organometallic vapor phase epitaxial growth and ordering in GaInP

CuPt ordering in GaInP has significant effects on the electrical and optical properties. In fact, band gap reductions as large as 160 meV are potentially useful for devices. Thus, control of ordering is important. This has led to the investigation of surfactants that affect the surface processes during growth with little incorporation into the solid. A potentially interesting class of surfactant elements are those from group VII, that are known not to be incorporated during growth, but to change the dynamics of surface processes. This article describes the results of the addition of Cl and Br during the organometallic vapor phase epitaxial growth of GaInP. The addition of Br in small concentrations is observed to systematically decrease the amount of CuPt ordering observed directly via transmission electron diffraction and photoluminescence measurements. A ratio of Br/III of 0.042 is found to virtually eliminate ordering. The reduction in order parameter is not found to correspond to a decrease in the 405 nm peak measured during in situ surface photoabsorption measurements. This indicates that a change in surface reconstruction is not responsible for the decrease in order parameter. However, both Cl and Br are found to significantly roughen the surface morphology. Atomic force microscopy shows that facets are formed that increase in height and angle to the (001) growth surface as the surfactant concentration increases. This is postulated to be the origin of the reduction in order parameter, since previous studies have indicated that an intentional misorientation of the substrate in either the A or B direction leads to a decrease in order parameter.

Organometallic vapor phase epitaxial growth of GaN on a 3c-SiC/Si(1 1 1) template formed by C +-ion implantation into Si(1 1 1) substrate

This paper reports a novel method to grow a wurtzite GaN on Si(1 1 1) by using a single-crystalline 3c-SiC interlayer. The interlayer is formed by the carbon (C +)-ion implantation into Si(1 1 1) wafers. The C +-ion implantation is performed at 600°C at an accelerating voltage of 180 kV. After the implantation wafers are annealed in O 2 atmosphere at 1250°C for 2 h. Auger analysis reveals the presence of SiC at a depth of 0.3–0.5 μm from the surface. Upper layers of SiO 2, Si and polycrystalline SiC on the single-crystalline 3c-SiC are successively removed and the single-crystalline SiC layer is successfully exposed on the surface. Organometallic vapor phase epitaxial (OMVPE) growth of GaN layer is made at 1000°C on the 3c-SiC/Si(1 1 1) template with a low-temperature (550°C) GaN buffer. The grown layer is confirmed to be single-crystalline wurtzite GaN by the RHEED pattern. The layer shows an intense excitonic emission with a peak energy of 3.416 eV at room temperature. The PL peak position and Raman shift show that the GaN layer has a small tensile stress of about 0.6 Gpa, which is a half of the value reported for a film grown on Si with an AlN buffer. This is supported by the fact that no cracks are found on the GaN surface.

Numerical analysis of 1.55‐µm wavelength optical deflector using arrayed waveguide with staircase‐like refractive index distribution

AbstractThis paper proposes an optical deflector using an arrayed waveguide with a staircase‐like refractive index profile. This device is an optical deflector using an array waveguide with the refractive index different in each of the component waveguides. Optical deflection is accomplished by application of a voltage. In order to study the deflection properties, a theoretical analysis of the structure, device parameters, deflection angle, and number of resolution points is carried out. As a method of realizing an arrayed waveguide with built‐in refractive index steps, selective organometallic vapor phase epitaxial growth is used for fabrication of an array waveguide with multiple quantum well structures. Initial experimental results are reported here. © 2003 Wiley Periodicals, Inc. Electron Comm Jpn Pt 2, 86(4): 1–9, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecjb.10103

Effects of Surfactants N and Br on Ordering in GaInP

ABSTRACTFormation of the CuPt ordered structure during the organometallic vapor phase epitaxial (OMVPE) growth of GaInP has significant effects on the electrical and optical properties, necessitating control of ordering. Formation of the CuPt structure is thermodynamically driven by the surface structure during growth. This has led to the study of the use of surfactants as a way of controlling ordering. To date, these studies have centered on the group V elements isoelectronic with P. Sb and Bi are both larger than P. This leads to less ordering. N, on the other hand, is smaller than P, which makes it potentially interesting. The other disordering mechanism, observed for Te as a surfactant in GaInP, is kinetic. Rapid step propagation leads to a reduction in ordering. This suggests another potentially interesting class of surfactants, the group VII elements, such as Br, that have been reported to increase the step velocity during OMVPE growth. This paper describes the results of the addition of the surfactants N and Br during the OMVPE growth of GaInP. Addition of N (DMHy) results in a clear reduction in order parameter, due to thermodynamic effects. The addition of Br (CBr4) is also observed to systematically decrease the amount of CuPt ordering and is found to correspond to a significant roughening of the surface, which is postulated to be the origin of the reduction in order parameter.

Alternative boron precursors for BGaAs epitaxy

The organometallic vapor phase epitaxy growth of zinc-blende BxGa1−xAs and BxGa1−x−yInyAs with boron concentrations (x) up to 3–5% has recently been demonstrated using diborane as a boron precursor. Growth of these alloys using diborane is complicated by many factors, such as parasitic gas-phase reactions, highly temperature-dependent boron incorporation, and limited boron incorporation before the onset of structural breakdown. These factors suggest that diborane may not be the best precursor for the growth of these alloys. We compare the use of alternative boron precursors; trimethylboron (TMB), triethylboron (TEB), and boron trifluoride (BF3), with diborane for the OMVPE growth of these boron containing III–V alloys. We find that TMB and BF3 do not result in significant boron incorporation into GaAs. TEB does result in boron incorporation in a manner very similar to diborane. Both diborane and TEB incorporate more efficiently using triethylgallium (TEG) rather than trimethylgallium (TMG), making TEG a preferred source of gallium for BGaAs epitaxy. Using TEB together with TEG, a higher boron composition (x=4–7%) has been achieved than has been previously reported, but the complicating problems observed with diborane still exist.

Enhancement of compositional modulation in GaInP epilayers by the addition of surfactants during organometallic vapor phase epitaxy growth

The use of surfactants in semiconductors during organometallic vapor phase epitaxy (OMVPE) is a powerful and exciting tool for controlling the properties of epilayers. The effects of the isoelectronic surfactants Sb and Bi have been systematically studied for GaInP alloys grown lattice-matched to GaAs substrates by OMVPE. Addition of small concentrations of either Sb or Bi leads to the elimination of the CuPt-B ordering observed in undoped layers. At high Sb concentrations, additional changes in the structure and optical properties are observed. The layers spontaneously form a structure with a lateral compositional modulation in the [1 1 0] direction. Surface undulations are clearly seen by atomic force microscopy (AFM) for the layers with compositional modulation. The low temperature photoluminescence (PL) peak energy is observed to decrease with increasing compositional modulation. The PL emission is strongly anisotropic, with a high intensity in the [ 1 ̄ 1 0] direction which is the reverse of the polarization observed for CuPt ordered GaInP layers. Compositional modulation and surface undulation are found to be strongly affected by the growth rate. The amplitude of the compositional modulation in GaInP layers grown with either surfactant is significantly increased for decreased growth rates. The process controlling the amplitude appears to be kinetically limited. It is likely that adding the surfactant Sb or Bi increases the ad-atom surface diffusion coefficients which acts to enhance the extent of compositional modulation in GaInP especially when grown at low rates.

OMVPE growth and properties of Dy-doped III–V semiconductors

Dy doping to GaAs and InP by organometallic vapor phase epitaxy (OMVPE) has been investigated, for the first time, with two kinds of Dy sources. Secondary ion mass spectroscopy (SIMS) measurements reveal a uniform Dy distribution along the growth direction. Good controllability in GaAs of Dy concentration is achieved against a H 2 flow rate through the Dy source. In 4.2 K photoluminescence (PL) measurements on Dy-doped GaAs, characteristic luminescence is successfully observed at around 1.1, 1.3, 1.7 and 2.8 μm , which is assigned to the intra-4f shell transition of Dy 3+ ions. The Dy-related PL intensity depends strongly on Dy concentration and growth conditions. In Dy-doped InP, weak Dy-related luminescence is also observed at around 1.3 μm .

Bi surfactant control of ordering and surface structure in GaInP grown by organometallic vapor phase epitaxy

The surfactant Bi has been added during organometallic vapor phase epitaxial growth (OMVPE) of GaInP using the precursor trimethylbismuth. The addition of a small amount of Bi during growth results in disordered material using conditions that would otherwise produce highly ordered GaInP. Significant changes in the surface structure are observed to accompany the disordering. Atomic force microscopy measurements show that Bi causes an order of magnitude increase in step velocity, leading to the complete elimination of three-dimensional islands for growth on singular (001) GaAs substrates, and a significant reduction in surface roughness. Surface photoabsorption measurements indicate that Bi reduces the number of [1̄10] P dimers on the surface. Secondary ion mass spectroscopy measurements reveal that the Bi is rejected from the bulk, even though it changes the surface reconstruction. Clearly, Bi acts as a surfactant during OMVPE growth of GaInP. The difference in band gap energy caused by the reduction in order parameter during growth is measured using photoluminescence to be about 110 meV for layers grown on singular substrates. Disorder/order/disorder heterostructures were successfully produced in GaInP with a constant solid composition by modulating the TMBi flow rate during growth.

Monolithic oxide-confined multiple-wavelength vertical-cavity surface-emitting laser arrays with a 57-nm wavelength grading range using an oxidized upper Bragg mirror

Monolithic, oxide-confined, multiple-wavelength vertical-cavity surface-emitting laser arrays with a very large periodic, wavelength grading span of 57 nm (from 968 to 1025 nm) have been achieved under room temperature, continuous-wave operation, with threshold currents of 4.5 mA/spl plusmn/1.5 mA. Almost linear wavelength grading is achieved by organometallic vapor phase epitaxial growth on a patterned substrate. An extended wavelength range is achieved by minimizing the optical loss dispersion by scaling the growth rate of all the epilayers and using a selectively-oxidized upper DBR mirror with a flattened optical reflectance spectrum, plus the higher differential optical gain provided by compressively-strained In/sub 0.2/Ga/sub 0.8/As-GaAs quantum wells.

The use of a surfactant (Sb) to induce triple period ordering in GaInP

A surfactant is used to induce an ordered structure in an epitaxial layer. The addition of small amounts of triethylantimony during the organometallic vapor phase epitaxy growth of GaInP on (001) GaAs substrates is shown to remove CuPt ordering with a resultant increase in band gap energy. Increasing the concentration of Sb in the vapor beyond a critical Sb to P ratio [Sb/P(v)] of 4×10−4 gives a reversal of this behavior. The band gap energy is observed to decrease by 50 meV at a concentration of Sb/P(v)=1.6×10−3, coincident with the formation of an ordered phase with a period triple the normal lattice spacing along the [111] and [1̄1̄1] directions. The formation of this new ordered structure is believed to be related to high concentrations of Sb on the surface, which leads to a change in the surface reconstruction from (2×4)-like to (2×3)-like, as indicated by surface photoabsorption performed in situ.