MOVPE of GaN using a specially designed two-flow horizontal reactor

GaN epilayers have been grown on (0001) sapphire substrates with a specially designed two-flow horizontal metalorganic vapor phase epitaxy (MOVPE) reactor. Epilayers with flat and smooth surfaces were obtained at the growth temperature of 950°C with relatively low source supply rates. This indicates a relatively high growth efficiency of the reactor. Characterization by photoluminescence, X-ray diffraction and Hall measurements reveal that the epilayers are of reasonably high quality.     

Computational studies of the transient behavior of horizontal MOVPE reactors

Dynamic models of the mass transfer in horizontal reactors used for metalorganic vapor phase epitaxy (MOVPE) of compound semiconductors have been developed and used to study the duration of transients on the substrate during the growth of heterostructures. Dispersion of reactants during gas switching can be detrimental to the abruptness of the interfaces. A solution to this problem is the operation of MOVPE reactors at very high flow rates and low pressures, but this leads to low conversions of the expensive precursors. Our simulations indicate that critical Péclet numbers can be identified, beyond which no significant reduction in the duration of transients on the substrate occurs when increasing the gas velocity. The substrate always reached a new steady state much faster than the entire reactor. Symmetry of filling and purging with respect to transients was also observed. Performance diagrams connecting optimal operating conditions with reactor geometry have been constructed. Time-dependent models of the MOVPE process can help identify optimal operating conditions and reactor shapes leading to abrupt interfaces with maximum precursor utilization.     

Effects of pressure and temperature on RD detected growth oscillations

We report on the vacuum chemical epitaxy (VCE) growth of GaAs from triethylgallium and arsine at varying partial pressures of arsine and hydrogen. In situ, monolayer growth oscillations were, for the first time, detected in a hydrogen environment using reflectance difference (RD). These results offer the possibility to link surface mechanisms occuring during chemical beam epitaxy (CBE) with those taking place in metalorganic vapour phase epitaxy (MOVPE) and may lead to the observation of growth oscillations also during MOVPE. Finally, the behaviour of the RD signal as a function of substrate temperature is studied over a wider temperature interval than has previously been reported, giving further information about surface processes.     

Growth monitoring by reflectance anisotropy spectroscopy in MOVPE reactors for device fabrication

Reflectance anisotropy spectroscopy (RAS) has proved its capability to study surface processes during metalorganic vapour phase epitaxy (MOVPE) growth of a variety of III–V compounds. However, these investigations up to now have been mostly restricted to specialized research reactors. Therefore, we studied the feasibility of in-situ monitoring by RAS during growth on two production-type MOVPE reactors: horizontal 2 inch single wafer reactor AIX 200 and Planetary Reactor™ AIX 2000 for 5 × 3 inch. The slight modifications of the reactors necessary to gain normal incidence optical access to the sample do not alter the properties of the grown materials. While in the horizontal reactor the strain-free optical window allows one to obtain well-resolved RAS spectra the signals in the multiwafer reactor are affected by the anisotropy of the ceiling plate. Even in this case RAS spectra can be extracted. First measurements on rotating samples in the horizontal reactor demonstrate the possibility to obtain RAS spectra by multitransient spectroscopy. As an application monitoring of the growth of p-type layers for the base of GaInP/GaAs hetero-bipolar-transistors (HBTs) is discussed. The linear electro-optic effect (LEO) gives information on doping type and doping level. Time-resolved transients at specific energies are used to study the impact of different switching schemes on the properties of the base-emitter interface.     

Metalorganic vapor phase epitaxy of III–V-on-silicon: Experiment and theory

The integration of III–V semiconductors with Si has been pursued for more than 25 years since it is strongly desired in various high-efficiency applications ranging from microelectronics to energy conversion. In the last decade, there have been tremendous advances in Si preparation in hydrogen-based metalorganic vapor phase epitaxy (MOVPE) environment, III–V nucleation and subsequent heteroepitaxial layer growth. Simultaneously, MOVPE itself took off in its triumphal course in solid state lighting production demonstrating its power as industrially relevant growth technique. Here, we review the recent progress in MOVPE growth of III–V-on-silicon heterostructures, preparation of the involved interfaces and fabrication of devices structures. We focus on a broad range of in situ, in system and ex situ characterization techniques. We highlight important contributions of density functional theory and kinetic growth simulations to a deeper understanding of growth phenomena and support of the experimental analysis. Besides new device concepts for planar heterostructures and the specific challenges of (001) interfaces, we also cover nano-dimensioned III–V structures, which are preferentially prepared on (111) surfaces and which emerged as veritable candidates for future optoelectronic devices.     

GaN/GaAs(1 0 0) superlattices grown by metalorganic vapor phase epitaxy using dimethylhydrazine precursor

Superlattices of cubic gallium nitride (GaN) and gallium arsenide (GaAs) were grown on GaAs(1 0 0) substrates using metalorganic vapor phase epitaxy (MOVPE) with dimethylhydrazine (DMHy) as nitrogen source. Structures grown at low temperatures with varying layer thicknesses were characterized using high resolution X-ray diffraction and atomic force microscopy. Several growth modes of GaAs on GaN were observed: step-edge, layer-by-layer 2D, and 3D island growth. A two-temperature growth process was found to yield good crystal quality and atomically flat surfaces. The results suggest that MOVPE-grown thin GaN layers may be applicable to novel GaAs heterostructure devices.     

Growth reactions and mechanisms in chemical beam epitaxy (CBE)

The rapidly increasing interest in chemical beam epitaxy (CBE) and related ultra-high vacuum (UHV)-based epitaxial growth techniques has arisen primarily as a result of their expected technological advantages compared to either the conventional molecular beam epitaxy (MBE) or metalorganic vapour phase epitaxy (MOVPE) processes. The CBE-related techniques do, however, provide the additional important advantage, compared to MOVPE, that the UHV environmental allows in-vacuo analytical techniques to be used to provide in-situ characterization regarding the composition and crystallography of the growing layers, and also vital information regarding the growth mechanisms involved. The present paper reviews the current understanding relating to III–V CBE reaction mechanisms, and highlights specific topics which require further investigation.     

Carry-over - A Fundamental Problem in the MOVPE Growth of Heterostructures

We have investigated the MOVPE growth of GaAs, GaP, and InP using TMGa-TMN (trimethylgallium-trimethylamine), TMIn-TMN (trimethylindium-trimethylamine) and the hydrides arsine and phosphine in reactor cells covered with products of preceding deposition processes. By measurements of the misfit Δa/a we find that besides arsenic also gallium and at higher growth temperatures even indium can be transferred from the predepositions into the epilayer. The mobilization of gallium is possible due to alkyl exchange reactions with indium-methyl-species. Indium is most probably mobilized due to its high vapour pressure at high growth temperatures. The extent of carry-over is limited within a range of a few percent impurity concentration.     

Effect of hetero-interfaces on in situ wafer curvature behavior in InGaAs/GaAsP strain-balanced MQWs

Using high-accuracy in situ curvature measurement during growth of InGaAs/GaAsP strain-compensated multiple quantum wells (MQWs) by metal organic vapor phase epitaxy (MOVPE), we have successfully clarified the effect of hetero-interfaces on strain control in InGaAs/GaAsP strain-balanced MQWs. By analyzing curvature transients and X-ray diffraction (XRD) fringe patterns, we found that an inadequate gas-switching sequence induces unintended atomic content at the interfaces between InGaAs and GaAsP and then influences the average strain of the structure. Through considering the atomic characteristics and measuring the reflectance anisotropy transient during growth, it has been revealed that the optimized stabilization time for arsenic and phosphorus mixture before GaAsP barrier growth should be longer than 3 s at 610 °C.     

Characterization of p-type ZnSe grown by MOVPE; excitonic emission lifetime measurements

Time-resolved photoluminescence (TRPL) measurements are made on p-type nitrogen-doped ZnSe grown by photoassisted metalorganic vapor phase epitaxy (MOVPE) together with post-growth thermal annealing, in order to investigate optical quality of the layers. It is suggested that the annealing degrades the layer quality and the MOVPE samples have more non-radiative recombination centers compared with MBE samples. A key issue for high quality p-ZnSe by MOVPE seems to be optimization of annealing conditions.     

MOVPE growth of GaInP/GaAs hetero-bipolar-transistors using CBr4 as carbon dopant source

Carbon doping of GaAs with carbon tetrabromide (CBr₄) in low pressure MOVPE has been investigated and applied to the fabrication of GaInP/GaAs HBTs. Especially the hydrogen incorporation and the associated acceptor passivation has been studied. The hydrogen found in single GaAs:C layers is predominantly incorporated during cooling the sample under AsH₃ after growth, n-Type capping layers can block this H indiffusion and GaAs:C base layers in HBTs show much lower H concentrations than GaAs:C single layers without a cap. A further reduction of acceptor passivation is possible by optimization of the growth procedure. First HBTs processed from layers with a base that was doped using CBr₄ show promising DC and HF performance (β = 45, for 2 × 20 μm² devices).     

Growth of InN films and nanostructures by MOVPE

Since a few years, a lot of research efforts have been devoted to InN, the least known of the semiconducting group-III nitrides. Most of the samples available today have been grown using the molecular beam epitaxy technique, and fewer using the metal organic vapor phase epitaxy (MOVPE) method. Whatever the method, the growth of InN is extremely challenging, in particular due to the fact that no lattice matched substrate is available.     

TEM investigations of GaN layers grown on silicon and sintered GaN nano‐ceramic substrates

GaN nano‐ceramics were analyzed using transmission electron microscopy (TEM), showing that these ceramics are characterized by highly disoriented grains of the linear size of 100–150 nm. These GaN ceramics were used as substrates for GaN epitaxy in standard MOVPE conditions. For the comparison, MOVPE GaN layers on silicon substrates were grown using similar conditions. It is shown that MOVPE growth of GaN layers is highly anisotropic for both cases. However, the disorientation of the highly mismatched GaN layer on silicon is different from that characterizing GaN layer deposited on the ceramic substrate. In the latter case the disorientation is much higher, and three dimensional in nature, causing creation of polycrystalline structure having large number of the dislocations. In the case of the GaN layer grown on the silicon substrate the principal disorientation is due to rotation around c‐axis, causing creation of mosaic structure of edge dislocations. Additionally, it is shown that the typical grain size in AlN nucleation layer on Si is smaller, of order of 20 nm. These two factors contribute to pronounced differences in later stage of the growth of GaN layer on the ceramic. Due to high growth anisotropy an appropriately thick GaN layer can, eventually, develop flat surfaces suitable for construction of optoelectronic and electronic structures. As shown by the TEM data, this can be achieved only at the cost of creation of the relatively large density of dislocations and stacking faults. The latter defects were not observed for the GaN growth on Si substrates. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Influence of substrate temperature on the shape of GaAs nanowires grown by Au-assisted MOVPE

GaAs nanowires (NWs) are grown on the GaAs(1 1 1)B substrates by the Au-assisted metal–organic vapor phase epitaxy (MOVPE). The NW shape is found to be strongly dependent on the substrate temperature during the growth. With increase in the growth temperature, the NW shape modifies from prismatic to conical. The observed temperature behavior is studied within the frame of a theoretical model. It is shown that the key process responsible for the lateral growth is the decomposition of MOVPE precursors at the NW sidewalls and the substrate. Theoretical results are in a good agreement with experimental findings and can be used for the numerical estimates of some important growth parameters as well as for the controlled fabrication of NWs with the desired shape.     

MOVPE-grown high CW power InGaAs/InGaAsP/InGaP diode lasers

Al-free InGaAs/InGaAsP/InGaP laser structures grown by MOVPE have yielded new records in performance. CW front-facet-emitted powers of 5.8 W are achieved from optimized double-quantum-well lasers with 100 μm wide stripes and 1 mm long cavity lengths. Devices with a 0.5 mm cavity length exhibit total power conversion efficiencies as high as 59%. A novel facet passivation technique, consisting of laser-assisted deposition of ZnSe, is shown to increase the COD level by 50%. Single-mode, CW output powers of 400 mW are obtained from triple-core ARROW lasers fabricated by a two-step MOVPE growth process.     

Process optimisation of MOVPE growth by numerical modelling of transport phenomena including thermal radiation

A global transport model for the MOVPE of III–V growth based on the finite volume solution of coupled flow, heat and mass transfer, including detailed radiative transfer, multicomponent diffusion and homogeneous and heterogeneous chemical reactions, is presented. For radiative transfer modelling, a combined approach is used of grey-diffuse view-factor based heat flux exchange between the semi-transparent reactor walls through the transparent reactor interior, and a spherical harmonics approximation for the radiative-conductive heat transfer problem in participating massive quartz elements with complex shapes. The described modelling approach is applied to the horizontal multiwafer radial flow Planetary Reactor™, validated experimentally and used for process improvements. The mutual interaction of changing radiation properties at internal solid boundaries due to semiconductor coatings and thermal behaviour in that particular MOVPE reactor is discussed.     

MOVPE of II–VI materials

The development of II–VI MOVPE is reviewed, contrasting the narrow bandgap materials with the wide bandgap. Common issues are the need to grow the layers at lower temperatures than their III–V cousins in order to avoid point defects. This means that II–VI MOVPE occurs in a surface kinetic regime for precursor decomposition and has stimulated a lot of research on alternative precursors. The narrow bandgap II–VI growers have settled on dimethyl cadmium (DMCd) combined with diisopropyl telluride (DIPTe) and a liquid Hg source but wide bandgap growers are split between pyrolytic and photo-assisted growth. Recent progress in p-type doping has enabled the demonstration of some new devices, including two colour infrared detectors and the first MOVPE grown green emitting laser structure. The common theme appears to be hydrogen passivation of the Group V dopant and some novel precursor solutions to this problem are discussed.     

Shape transition from InAs quantum dash to quantum dot on InP(3 1 1)A

We report on the shape transition from InAs quantum dashes to quantum dots (QDs) on lattice-matched GaInAsP on InP(3 1 1)A substrates. InAs quantum dashes develop during chemical-beam epitaxy of 3.2 monolayers InAs, which transform into round InAs QDs by introducing a growth interruption without arsenic flux after InAs deposition. The shape transition is solely attributed to surface properties, i.e., increase of the surface energy and symmetry under arsenic deficient conditions. The round QD shape is maintained during subsequent GaInAsP overgrowth because the reversed shape transition from dot to dash is kinetically hindered by the decreased ad-atom diffusion under arsenic flux.     

Characterization of GaN using thermally stimulated current and photocurrent spectroscopies and its application to UV detectors

Deep levels in insulating GaN grown by metalorganic vapor phase epitaxy (MOVPE) have been studied using thermally stimulated current (TSC) and photocurrent (PC) spectroscopies. Five main levels (0.11, 0.24, 0.36, 0.53 and 0.62 eV) were observed by TSC measurements in the as-grown undoped GaN. PC measurements showed three deep levels located within bandgap at 1.32, 1.70 and 2.36 eV, respectively. We found that three of the levels, located at 0.24, 0.36 and 0.53 eV, were eliminated by annealing at 1000°C under N₂ for six hours, whereas the 0.62 eV level density increased after annealing. In addition, both the responsivity and on/off times of GaN metal-semiconductor-metal (M---S---M) detectors degrade with increasing concentration of the 0.62 eV trap. We have also found that this trap can be effectively reduced by increasing the ammonia flow rate during the MOVPE growth. Accordingly, a high responsivity ( 3200 A/W) UV detector with an improved response time, from 8 to 0.4 ms, was fabricated on GaN grown under the optimized conditions.     

Low-temperature molecular beam epitaxy of GaAs: Influence of crystallization conditions on structure and properties of layers

A nontraditional approach to the control of GaAs properties via the introduction of an excessive amount of arsenic during growth of epitaxial layers under conditions of low-temperature molecular-beam epitaxy (LT-GaAs layers) is considered. The influence of excessive arsenic on the structure and properties of as-grown and annealed LT-GaAs layers is considered as well as the effect of layer doping on the “capture” of excess arsenic.