In situ processing of III–V semiconductors: Mile stones and future prospects

In situ processing combined with metalorganic vapor phase epitaxy (MOVPE), molecular beam epitaxy, or chemical beam epitaxy appears to be an attractive method for fabricating sophisticated optoelectronic devices such as buried heterostructure lasers, vertical cavity surface emitting lasers, and photonic integrated circuits. Successful reduction of residual contaminants at the regrowth interface and improvement in the optical and electrical quality of the regrown layer has been achieved by using in situ processing techniques. Device fabrication is alrady taking advantage of this kind of technology. Nevertheless, interface quality between an in situ etched layer and a regrown layer has not yet reached the status of continuously grown interfaces. In this paper, progress of in situ processing is reviewed mainly focusing on our recent studies on in situ HCl gas etching in MOVPE. The approach of two-step HCI gas etching has proven superior to obtain clean regrowth interfaces, leading to the conclusion that the in situ processing can be widely used for advanced optoelectronic device fabrication.     

Alternative precursors for MOVPE growth of InN and GaN at low temperature

We report on the use of dimethylhydrazine (DMHy) and tertiarybuthylhydrazine (TBHy), as alternative nitrogen precursor for GaN low-temperature growth, as well as to improve the InN growth rate. Lowering the GaN growth temperature, would allow growing InN/GaN heterostructures by MOVPE, without damaging the InN layers. Increasing the low InN MOVPE growth rate is of major importance to grow reasonably thick InN layers. In this respect, triethylindium (TEIn) was also used as an alternative to trimethylindium (TMIn).     

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.     

High quality GaN grown by MOVPE

Several nitrogen precursors have been used for the growth of GaN in MOVPE, but so far the best results were obtained using NH₃, even though NH₃ does not produce a significant amount of active species at the growing interface. To produce active species from N₂ or NH₃, a remote plasma-enhanced chemical vapour deposition (RPECVD) process has been implemented. In addition, nitrogen metalorganic precursors, triethylamine and t-butylamine, were also used. To accurately control the critical parameters of the MOVPE of GaN, we have implemented a laser reflectometry equipment, which allows a real-time in situ monitoring of the different steps of the growth, i.e. nitridation of the substrate, nucleation, heat treatment, and deposition. Using an appropriate buffer layer, GaN grown on sapphire using NH₃ as nitrogen precursor, shows sharp low temperature photoluminescence lines (4 meV at 9 K), whereas other nitrogen precursors did not lead to comparable electronic quality.     

AlGaN/GaN HEMTs regrown by MBE on epi-ready semi-insulating GaN-on-sapphire with inhibited interface contamination

In this work, we show that, by carefully designing the subsurface Fe doping profile in SI-GaN templates grown by MOVPE and by optimizing the MBE regrowth conditions, a highly resistive GaN buffer can be achieved on these epi-ready GaN-on-sapphire templates without any addition of acceptors during the regrowth. As a result, high-quality high electron mobility transistors can be fabricated. Furthermore, we report on the excellent properties of two-dimensional electron gas and device performances with electron mobility greater than 2000 cm²/V s at room temperature and off-state buffer leakage currents as low as 5 μA/mm at 100 V.     

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.     

Metalorganic chemical vapor epitaxy and doping of ZnMgSSe heterostructures for blue emitting devices

Blue-green semiconductor laser diodes operating at room temperature are still the domain of wide bandgap II–VI compound semiconductors. CW operation at room temperature and hours of lifetime were reported. However, the conductivity control, defect generation and the ohmic contacts still need improvement. Therefore we focused our work on the MOVPE growth and the optimization of ZnMgSSe/ZnSSe/ZnSe heterostructures as well as on nitrogen doping of ZnSe. To verify the layer quality characterization was carried out by X-ray diffraction, electron probe microanalysis, electrical measurements and photoluminescence. ZnMgSSe/ZnSSe/ZnSe and ZnSSe/ZnSe quantum wells and superlattices were grown to investigate structural as well as interface properties. Electron beam and optical pumping was used to clarify the laser mechanism and to clarify the suitability of a MOVPE process to grow laser quality material. The electrical compensation of ZnSe doped with nitrogen is still controversially discussed whereas high n-type doping with chlorine was reproducible achieved. ZnSe: N doped at different growth conditions (II/VI ratio, growth temperature, nitrogen supply) using N₂ excited in a plasma source or by the use of nitrogen containing precursors was investigated to study the compensation mechanisms.     

Correlation of ordering formation and surface structure in (GaIn)P using modulated MOVPE

Modulated metalorganic vapour phase epitaxial growth (MOVPE) is used to clarify the role of the surface conditions on the ordering behaviour in ternary (GaIn)P layers. The alternating deposition of GaP and InP layers with individual thicknesses of up to one monolayer is successfully used for the growth of (GaIn)P bulk layers lattice matched to (100) GaAs substrates with various off-orientations. The layer quality and the degree of ordering are investigated using high-resolution X-ray diffraction (XRD), transmission electron microscopy (TEM), and photoluminescence spectroscopy (PL), respectively. The application of modulated growth conditions for the deposition of (GaIn)P bulk layers has a strong influence on the degree of ordering achieved in the intermediate growth temperature regime where the highest degree of ordering occurs under continuous MOVPE. Beside a new boundary structure observed in layers grown under modulated flux conditions, the successful growth of highly ordered (GaIn)P layers grown using the modulated MOVPE technique support the model that up to 2 monolayers of the (GaIn)P growth surface are involved in the ordering formation process.     

Defect reduction in GaN epilayers and HFET structures grown on (0 0 0 1)sapphire by ammonia MBE

The quality of GaN epilayers grown by molecular beam epitaxy on substrates such as sapphire and silicon carbide has improved considerably over the past few years and in fact now produces AlGaN/GaN HEMT devices with characteristics among the best reported for any growth technique. However, only recently has the bulk defect density of MBE grown GaN achieved levels comparable to that obtained by MOVPE and with a comparable level of electrical performance. In this paper, we report the ammonia-MBE growth of GaN epilayers and HFET structures on (0 0 0 1)sapphire. The effect of growth temperature on the defect density of single GaN layers and the effect of an insulating carbon doped layer on the defect density of an overgrown channel layer in the HFET structures is reported. The quality of the epilayers has been studied using Hall effect and the defect density using TEM, SEM and wet etching. The growth of an insulating carbon-doped buffer layer followed by an undoped GaN channel layer results in a defect density in the channel layer of 2×10⁸ cm⁻². Mobilities close to 490 cm²/Vs at a carrier density of 8×10¹⁶ cm⁻³ for a 0.4 μm thick channel layer has been observed. Growth temperature is one of the most critical parameters for achieving this low defect density both in the bulk layers and the FET structures. Photo-chemical wet etching has been used to reveal the defect structure in these layers.     

In situ optical monitoring of AlGaN thickness and composition during MOVPE growth of AlGaN/GaN microwave HFETs

Real-time spectral reflectometry has been implemented to monitor the MOVPE growth of AlGaN/GaN microwave HFET structures. The aim is to monitor and control the thickness and composition of the thin AlGaN layer during growth. In order to extract useful information from the in situ spectra the optical constants of AlGaN as a function of alloy composition are required at the growth temperature (1050°C). As the first step to obtaining the high temperature optical constants, a room temperature spectroscopic ellipsometry study (energy range 1.65–4.95 eV) has been carried out on thin AlGaN films of various thickness (30 and 100 nm) and aluminium content (0.15 and 0.25). The multilayer model of each sample from the ellipsometry study is used to generate a reflectance spectrum which is compared with the in situ spectral reflectometry spectrum of the same sample acquired at room temperature to verify the technique. Further work is in progress to model the bandgap and optical constants of GaN and AlGaN at growth temperature.     

Progress in Modeling of III-Nitride MOVPE

This review provides an introduction to III-Nitrides MOVPE process modeling and its application to the design and optimization of MOVPE processes. Fundamentals of the MOVPE process with emphasis on transport phenomena are covered. Numerical techniques to obtain solutions for the underlying governing equations are discussed, as well as approaches to describe multi-component diffusion for typical regimes during MOVPE. Properties of common industrial MOVPE reactor types like close spaced showerhead reactors, rotating disk reactors and Planetary Reactors are compared in terms of underlying working principles and generic process parameter dependencies.The main part of the paper is devoted to reviewing gas phase and surface reaction mechanisms during MOVPE. The process design in particular for MOVPE of III-Nitrides is determined by complex gas phase reaction kinetics. Advances in the modeling and predicting of these processes have contributed to understanding and controlling these phenomena in industrial scale MOVPE reactors. Detailed kinetics and simplified surface kinetic approaches describing the incorporation of constituents into multinary solid alloys are compared and a few application cases are presented. Differences in thermodynamic and kinetic properties of multi-layered structures of different compositions such as InGaN, AlGaN can cause enrichment of the adsorbed layer by certain group III atoms (indium in case of InGaN and gallium in case of AlGaN) that translate into specific features of composition profiles along the growth direction.An intrinsic feature of III-nitride materials is epitaxial strain that shows up in different forms during growth and affects both deposition kinetics and material quality. In case of InGaN MOVPE there is a strong interplay between indium content and strain that has direct influence on distribution of material composition in the epitaxial layers and multi-layered structures. Epitaxial strain can relax via different routes such as nucleation and evolution of the extended defects (dislocations), layer cracking and roughening of the surface morphology. Simulation approaches that address coupling of growth kinetics with strain and defect dynamics are discussed and exemplified.     

HigHighly strained InGaAs/GaAs quantum wells emitting beyond 1.2 µm

Highly strained In_xGa_1–xAs quantum wells (QWs) with GaAs barriers emitting around 1.2 µm are grown on GaAs substrates by metal organic vapour phase epitaxy (MOVPE) at low growth temperatures using conventional precursors. The effects of growth temperature, V/III ratio and growth rate on QW composition and luminescence properties are studied. The variation of indium incorporation with V/III ratio at a growth temperature of 510 °C is found to be opposite to the results reported for 700 °C. By an appropriate choice of the growth parameters, we could extend the room temperature photoluminescence (PL) wavelength of InGaAs/GaAs QWs up to about 1.24 µm which corresponds to an average indium content of 41% in the QW. The results of the growth study were applied to broad area laser diodes emitting at 1193 nm with low threshold current densities. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

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.     

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.     

Large scale manufacturing of compound semiconductors by MOVPE

As more compound semiconductor devices reach large volume manufacturing levels, a trend toward the use of the MOVPE technique is clear. In this paper we examine the criteria needed for MOVPE equipment suitable for large scale manufacturing. We find that although uniformity and device performance are necessary, reproducibility is also critical, along with high throughput and low operating costs. These points are illustrated by actual examples including MMIC power amplifiers, HB-LEDs, and solar cells. A realistic COO model provides a tool for evaluating MOVPE systems of different capacities. In situ control of key parameters during growth is now feasible, and will become an important method for increasing reproducibility and throughput. Lastly we look at the prospects for automation, for decreasing labor costs as well as wafer handling. This is likely to first have an impact on systems for the growth of electronic device structures on large (100 and 150 mm) wafers.     

Highly uniform growth in a low-pressure MOVPE multiple wafer system

A novel horizontal metal organic vapor phase epitaxy (MOVPE) system, which is capable of handling six 3 inch wafers or eighteen 2 inch wafers mounted on a 10 inch diameter susceptor, has been developed for the growth of III–V compound semiconductors. The characteristic features in this system are “triple flow channel” gas injection and “face-down” wafer setting configuration. The inlet for the source gas flow is divided into three zones (upper, middle and lower flows for hydrides, organometals and hydrogen, respectively) to control the concentration boundary layer and the growth area. The wafers are placed inversely to prevent thermal convection and particles on the growing surface. The independent controlled three-part heating system is also adopted to achieve a uniform temperature distribution over an 8 inch growing surface. The thickness and the doping of GaAs, Al_0.3Ga_0.7As, In_0.48Ga_0.52P and In_0.2Ga_0.8As grown by this system are uniform within ± 2% over all 3 inch wafers.     

Anisotropy in InGaAs/GaAs heterostructures grown by low-pressure MOVPE and CBE

InGaAs/GaAs heterostructures grown on (001) substrates by low-pressure MOVPE exhibit a measurable anisotropy in their structural, optical and electrical properties. This anisotropy occurs in structures which have undergone partial or complete strain relaxation and it can be strongly reduced by using slightly misoriented substrates. A comparison with similar structures grown by CBE indicates that this anisotropy is less important. This study suggests that strain relaxation is achieved by a combination of several mechanisms whose relative importance depends on the orientation of the substrate and on growth temperature which varies with the growth technique.     

Dependence of impurity incorporation upon substrate misorientation during GaAs growth by metalorganic vapour phase epitaxy

Doping studies of the incorporation behaviour of three different dopants (Zn, In and Si) versus the misorientation of the (100) surface during MOVPE growth of GaAs have been carried out with diethylzinc, trimethylindium and disilane as precursors. The incorporation of the dopants has been studied as function of the input mole fraction dopant, growth temperature, degree and direction of misorientation. In order to explain the results we discuss the BCF theory and the nature of the steps as function of above mentioned parameters. It appears that the BCF theory alone cannot explain the results, a counteracting mechanism has been introduced based on preferential arsenic desorption from the step edges.     

Thermodynamic analysis of the MOVPE growth of InxGa1−xN

A chemical equilibrium model is applied to the growth of the In_xGa_1−xN alloy grown by metalorganic vapor-phase epitaxy (MOVPE). The equilibrium partial pressures and the phase diagram of deposition are calculated for the In_xGa_1−xN alloy. The vapor-solid distribution relationship is discussed in comparison with the experimental data reported in the literature. It is shown that the solid composition of the In_xGa_1−xN alloy grown by MOVPE is thermodynamically controlled and that the incorporation of group III elements into the solid phase deviates from a linear function of the input mole ratio of the group III metalorganic sources under the conditions of high mole fraction of decomposed NH₃ (high value of ), high temperature and low input V/III ratio. The origin of the deviation of the solid composition from the linear relation is also discussed.     

Behaviour of vicinal InP surfaces grown by MOVPE: exploitation of AFM images

InP substrates and epilayers grown by MOVPE have been studied by AFM. For different misorientation angles, we observed the surface of the substrate after annealing under phosphine (PH₃) and of the epilayers under different growth conditions such as growth temperature T_g and trimethylindium (TMI) partial pressure. After annealing the terrace width corresponds to the nominal value of misorientation angle measured by X-ray diffraction. We observed different topographies and roughnesses for the grown layers corresponding to different growth modes. We propose, taking into account the roughness of the surface, a calculation of the step height and terrace width. For 2D nucleation (θ ≤ 0.2° and T_g = 500°C) and step flow mode, the roughness is low while it is increased by step bunching (θ ≥ 0.5° and T_g ≥ 580°C). Moreover we have examined the surface morphology for different misorientation angles and determined the influence of growth conditions (growth temperature, indium partial pressure) on the growth mechanism. At T_g = 580°C the increase of the TMI partial pressure in the reactor enhances the step bunching and leads to larger terraces.