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.     

High performance deep red AlAs/AlGaAs top-emitting VCSELs grown by MOVPE at high growth rates

Top-emitting Alas/AlGaAs vertical cavity surface emitting lasers emitting at 765 nm with minimum threshold currents of 0.6 mA and threshold voltages of 1.9 V have been grown by MOVPE. In order to keep the growth time low, we investigated the possibility to grow these structures at growth rates of 5 μm/h. Special attention was paid to the homogeneity that can be achieved over a 2″ wafer under these growth conditions. Spatially resolved reflectivity measurements on GaAs/AlAs distributed Bragg reflectors showed, that the growth rate varies less than 0.3% in the center of the wafer and decreases by 1% at the wafer edge.     

Microampere laser threshold at 80°C with InGaAs/GaAs/InGaP buried heterostructre strained quantum well lasers

An extremely low CW threshold current of 670 μA and a high slope efficiency of 0.14 W/A at a high junction temperature of 80°C were obtained with a 200 μm long Al-free InGaAs/GaAs/InGaP buried heterostructure (BH) quantum well laser grown by three-step metal organic vapor phase epitaxy (MOVPE). The maximum energy conversion efficiency of a 500 μm long laser was as high as 50% at a output power level of 1 mW. Regrowth conditions of InGaP layers were found to be crucial for planarizing the grown surface to realize the high performances.     

Visible (683 to 713 nm) room-temperature AlGaAs vertical cavity surface emitting lasers (VCSELs)

We compare the materials parameters for visible emitting, vertical cavity surface emitting lasers (VCSELs) and conclude that AlGaAs-based materials are a good choice. All-AlGaAs devices were produced by metalorganic vapour phase epitaxy (MOVPE) growth using ultra-high purity source reagents. Lasing was obtained at wavelengths in the range of 683 to 713 nm using four 45 Å wide Al_0.18Ga_0.82 As quantum wells in the active region. A threshold current density of 3.8 kA cm⁻² was measured for a cavity wavelength of 692 nm at room temperature. Growth of the epitaxial mirrors at 5.2 μm/h results in a total device growth time of only two and a half hours.     

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.     

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.     

Nonlinear Optics of Nematic and Blue Phase Liquid Crystals

We present a critical reexamination of individual electronic and collective optical nonlinearities of nematic liquid crystals (NLC) in the context of all-optical transmission switching of femtoseconds – CW lasers, and report recent observations of similar optical nonlinearities in Blue-phase liquid crystals (BPLC) and their utilization in feasibility demonstrations of polarization-free nonlinear grating diffraction and transmission switching of CW as well as pulsed lasers.     

Highly uniform AlGaAs/GaAs and InGaAs(P)/InP structures grown in a multiwafer vertical rotating susceptor MOVPE reactor

Highly uniform AlGaAs/GaAs and InGaAs(P)/InP epitaxial layers have been grown in a vertical rotating susceptor MOVPE reactor capable of accommodating three 2′ wafers. The unique water-cooled “showerhead”-type injection distributor which is located 1.5 cm above the substrates ensures a uniform reactant distribution, resulting in uniform growth over a wide range of growth conditions. Periodic multilayer and single layer structures have been used to investigate the thickness and compositional uniformities. The thickness variations over a radial distance of 48 mm for three wafers grown in the same run are within ± 2% for both AlGaAs and InGaAs layers, resulting in a standard deviation of only 0.9%. The gallium concentration of an InGaAs layer varies from 46.88% to 47.01% over the same radial distance with the standard deviation of 0.043%. Measurements of InGaAsP layers grown onto 2′ InP wafers with different alloy compositions show good compositional uniformity yielding standard deviations within 4.4 nm in PL wavelength and 135 ppm in lattice mismatch over a 46 mm radial distance.     

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.     

Growth of 1.55 μm DH laserstructures using TBAs and TBP in MOMBE

In a comparative study we have chosen TBAs and TBP as well as AsH₃ and PH₃ for the growth of InP/GaInAs(P) heterostructures for laser applications in a production metalorganic molecular beam epitaxy (MOMBE) system. The n-type doping was performed with Si from an effusion cell, whereas for the p-type doping Be and DEZn were utilized. InP layers using TBP under optimized cracking conditions exhibit excellent surface morphology with good electrical properties in the low 10¹⁵ cm⁻³ range of carrier concentrations. The MOMBE growth mechanism is not disturbed by the hydride replacement compound. This allows for a convenient replacement without losing calibration data from the hydride process. Broad-area DH laserstructures with GaInAsP (λ = 1.55 μm) active regions were grown with AsH₃/PH₃ and TBAs/TBP. Comparable threshold current densities in the range of 1.6-2.3 kA/cm² are achieved for the lasers, grown with both sets of precursors combined with DEZn source doping. These results are in good agreement with the standard set by the hydride MOVPE process.     

GaAs---InP heteroepitaxy and GaAs---InP MESFET fabrication by MOVPE

Lattice-mismatched heteroepitaxy has attracted considerable attention in recent years. A great interest of these systems is the possibility of integrating devices from different materials on a single substrate. 1.3 and 1.5 μm InGaAs(P)/InP laser diodes are essential for optical communication, whereas InP field effect transistor technology is less developed than that of GaAs MESFET. The performances of laser diodes are much more sensitive to a high density of disclocations, so it would be interesting to grow GaAs MESFET on InP for integration with 1.3 and 1.5 μm lasers. Due to the large difference of the thermal expansion coefficient and lattice parameter between GaAs and InP, it is very difficult to grow GaAs epilayers of high quality on it is very difficult to grow of GaAs epilayer high quality on InP substrates due to the large difference of the thermal expansion coefficient and lattice parameter between GaAs and InP. A new method, metalorganic source modulation epitaxy (MOSME), which improves the crystal quality of GaAs epilayers on InP substrates by MOVPE, has been adopted in our laboratory. The lowest full width at half maximum (FWHM) of the double crystal X-ray (DCX) diffraction spectra reaches as low as 120 arcsec for a 5 μm thick layer. Structural properties (misorientation, lattice parameters and crystal quality) of 1.0–5.0 μm thick GaAs layers grown on InP have been measured by DCX diffraction. On GaAs MESFETs grown on InP, we have measured g_m = 100 ms/mm. For these transitions, the current gain cut-off frequency (F_t) is around 12 GHz and the maximum frequency of oscillation (F_max) is higher than 30 GHz.     

Low-temperature LPE technique for the performance of visible AlGaAs (SC) lasers

The possibilities of the low-temperature (LT)-LPE technique in the performance of visible AlGaAs laser diodes are studied. Low current density visible lasers λ ˜ 746 nm has been fabricated and characterized. The laser diode structures are of the separate confinement type (SC) to obtain low threshold current (J_th = 190 A/cm² at λ = 848 nm for a cavity length L = 1.15 mm). The threshold current density J_th versus lasing wavelength λ characteristic, as well as the dependences of the threshold current density J_th and the reciprocal of differential quantum efficiency 1/η_d with the cavity length L are reported. The expected linear behaviour of the J_th and 1/η_d with the cavity length characteristics appears to break down for the shortest cavity length which is larger for shorter lasing wavelengths.     

Industrial production of GaN and InGaN-light emitting diodes on SiC-substrates

Blue light emitting diodes (LEDs) based on GaN or InGaN have a large market, e.g. for communication, industry and automotive applications. Since 1998, OSRAM Opto Semiconductors has been producing blue LEDs on a large scale, concentrating on the automotive market. LEDs in the wavelength range 450–480 nm are grown by metalorganic vapour phase epitaxy (MOVPE) on SiC. This substrate material offers many advantages from the epitaxial and device processing points of view. To fulfil the strong consumer recommendations not only do the epitaxial processes have to be developed, but also improvements in chip technology and package design help to stabilise electrical and optical properties on these high level demands. For example, low forward voltages, high light output powers at 20 mA, low reverse currents, long term stability and high electrostatic discharge robustness have to be guaranteed in a temperature range of −55–+85°C and at a maximum humidity of 85%. Electrical and optical parameters were tested on every produced chip not only to remove LEDs before packaging, which do not meet the specifications, but to control and optimise the applied technological steps. As an example we demonstrate a testing method for controlling the process and optimising the p-contact.Optimisation of n- and p-contact as well as the improvement of epitaxy helped to increase the light output power from 1 mW in 1998 to more than 6 mW at present when mounted in a 5 mm radial lamp. All electrical parameters of these high brightness LEDs could also be improved or at least kept in the demanded specification range.     

InP/InGaAlAs distributed Bragg reflectors grown by low-pressure metal organic chemical vapor deposition

Long-wavelength vertical cavity surface emitting lasers (VCSELs) are considered the best candidate for the future low-cost reliable light sources in fiber communications. However, the absence of high refractive index contrast in InP-lattice-matched materials impeded the development of 1.3–1.5 μm VCSELs. Although wafer fusions provided the alternative approaches to integrate the InP-based gain materials with the GaAs/AlAs materials for their inherent high refractive index contrast, the monolithic InP-based lattice-matched distributed Bragg reflectors (DBRs) are still highly attractive and desirable. In this report, we demonstrate InP/InGaAlAs DBRs with larger refractive index contrast than InP/InGaAsP and InAlAs/InGaAlAs DBRs. The switching between InP and InGaAlAs layers and growth rate control have been done by careful growth interruption technique and accurate in situ optical monitoring in low-pressure metal organic chemical vapor deposition. A 35 pairs 1.55 μm centered InP/InGaAlAs DBRs has the stopband of more than 100 nm and the highest reflectivity of more than 99%. A VCSEL structure incorporating 35 pairs InP/InGaAlAs DBR as the bottom mirror combined with a 2λ thick periodic gain cavity and 10 pairs SiO₂/TiO₂ top dielectric mirrors was fabricated. The VCSELs lased at 1.56 μm by optical pumping at room temperature with the threshold pumping power of 30 mW.     

Strain-compensated quantum cascade lasers operating at room temperature

Quantum cascade (QC) lasers based on strain-compensated In_xGa_(1−x)As/In_yAl_(1−y)As grown on InP substrate using molecular beam epitaxy is reported. The epitaxial quality is demonstrated by the abundant narrow satellite peaks of double-crystal X-ray diffraction and cross-section transmission electron microscopy of the QC laser wafer. Laser action in quasi-continuous wave operation is achieved at λ≈3.6–3.7μm at room temperature (34°C) for 20 μm×1.6 mm devices, with peak output powers of 10.6 mW and threshold current density of 2.7 kA/cm² at this 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.     

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.     

MOVPE-grown (AlGa)As double-barrier multiquantum well (DBMQW) laser diode with low vertical beam divergence

We present a (AlGa)As laser diode based on a MOVPE-grown heterostructure modified toward reduction of the vertical (perpendicular to junction plane) light beam divergence. Insertion of thin, wide-gap barrier layers at the interfaces between the MQW active region and the cladding layers allows for separate controlling the carrier and optical confinements. According to theoretical modeling, the antiguiding influence of the barriers on the primary waveguiding properties of a MQW structure causes a weakening of the optical confinement and thereby a reduction of the vertical beam divergence. This occurs however without weakening of the carrier confinement and an excessive increase of threshold current density. As a result, the beam divergence of 17-13° has been experimentally achieved (depending on construction details), for devices manufactured from the heterostructure theoretically expected to give 12°. MOVPE growth conditions of this modified “double-barrier multiquantum well” (DBMQW) heterostructure are described. The light-current characteristics presented show threshold current densities of 0.92-2.1 kA cm⁻² for laser cavity lengths of 0.75-0.25 mm, respectively and good quantum efficiencies.     

Low-threshold strained multi-quantum well lasers fabricated by selective metalorganic vapor phase epitaxy without a semiconductor etching process

Strained InGaAsP multi-quantum well (MQW) buried hetero- (BH) laser diodes (LDs) on a p-InP substrate were fabricated by selective metalorganic vapor phase epitaxy (MOVPE). In the laser fabrication process, both the strained MQW active layer and current blocking structure were directly formed by selective MOVPE without a semiconductor etching process. This novel laser fabrication process produces extremely uniform device characteristics that are essential to the deployment of optical subscriber systems. Furthermore, important device design parameters (e.g. the active stripe shape or the leakage current path configuration) are precisely controlled by only the epitaxial growth steps. This highly controllable laser fabrication method results in a very low-threshold current with excellent uniformity (I_th = 1.78 ± 0.19 mA) for 20 consecutive LDs (L = 200 μm with 70%–90% coatings).     

GaN-based lasers on SiC: influence of mirror reflectivity on L–I characteristics

The influence of the mirror reflectivity on the L–I characteristics of GaN-based lasers has been studied. A cleaved, Al-coated fiber is used as an external micro-mirror to control the reflectance of the end facets allowing for a continuous adjustment of mirror losses of a particular laser. In contrast to other methods, this eliminates all ambiguities usually arising from the comparison of different or differently coated devices. An increase in the single facet external quantum efficiency by 45% is observed for uncoated lasers and simultaneously, the threshold current is reduced by 12%. Internal losses of approximately 20–30 cm⁻¹ are derived from the differential quantum efficiency variation depending on the particular device under investigation.