Epitaxy of High-Power Diode Laser Structures (original) (raw)
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Optimization of VGF-growth of GaAs crystals by the aid of numerical modelling
Journal of Crystal Growth, 2002
The VGF growth of Si-doped GaAs crystals is improved considerably by optimizing the design of the crucible support and the temperature profile during the growth run. Inverse simulation with the software program CrysVUN++ was used for this procedure. The criteria for the optimized conditions are flat phase boundaries and low thermal stress during the whole growth run. The crystals which were grown according to the simulated conditions indeed showed flat phase boundaries and a very low EPD (o100 cm À2 ) within the whole crystal. It is shown that the growth conditions in the seed well and conical part of the crystal have a major influence on the dislocation density in the whole crystal. r (G. M . uller). 0022-0248/02/$ -see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 -0 2 4 8 ( 0 1 ) 0 2 3 2 4 -7 G. M . uller, B.
GaAs Substrates for High-Power Diode Lasers
Topics in Applied Physics, 2000
GaAs substrate crystals with low dislocation density (Etch-Pit Density (EPD) < 500 cm −2) and Si-doping (≈ 10 18 cm −3) are required for the epitaxial production of high-power diode-lasers. Large-size wafers (≥ 3 in) are needed for reducing the manufacturing costs. These requirements can be fulfilled by the Vertical Bridgman (VB) and Vertical Gradient Freeze (VGF) techniques. For that purpose we have developed proper VB/VGF furnaces and optimized the thermal as well as the physico-chemical process conditions. This was strongly supported by extensive numerical process simulation. The modeling of the VGF furnaces and processes was made by using a new computer code called CrysVUN++, which was recently developed in the Crystal Growth Laboratory in Erlangen. GaAs crystals with diameters of 2 and 3 in were grown in pyrolytic Boron Nitride (pBN) crucibles having a small-diameter seed section and a conical part. Boric oxide was used to fully encapsulate the crystal and the melt. An initial silicon content in the GaAs melt of c(Si melt) = 3 × 10 19 cm −3 has to be used in order to achieve a carrier concentration of n = (0.8-2)×10 18 cm −3 , which is the substrate specification of the device manufacturer of the diode-laser. The EPD could be reduced to values between 500 cm −2 and 50 cm −2 with a Si-doping level of 8 × 10 17 to 1 × 10 18 cm −3. Even the 3 in wafers have rather large dislocation-free areas. The lowest EPDs (< 100 cm −2) are achieved for long seed wells of the crucible. The fabrication of high-power diode-lasers requires GaAs substrates with special material and surface properties for the epitaxial processing. The mostimportant requirement is a low dislocation density expressed in terms of the Etch-pit density (EPD), with a value of EPD < 500 cm −2. This low EPD is an indispensable prerequisite for diode-laser fabrication as dislocations are deleterious for the lifetime and performance of the devices. The n-type electrical conduction (achieved by Si doping) with a carrier concentration of about 10 18 cm −3 is necessary because the substrate forms one of the diode contacts. Other important properties of the substrate wafers like orientation, surface and mechanical properties are discussed in Sect. 4.
Journal of Crystal Growth, 1999
GaAs single crystals (silicon-doped, 2 inch) with extremely low dislocation densities (etch pit density (EPD) 50-1000 cm\) were grown by the vertical gradient freeze method. In these crystals we characterised different types of dislocations by the aid of white beam X-ray diffraction topography and infrared transmission microscopy. It was found for decreasing dislocation densities (EPD(200 cm\), that dislocations having a line vector l, which is parallel to the [0 0 1] growth direction of the crystals, become more and more dominant. These residual dislocations are induced by the seeding process (so far we were using LEC-grown seed crystals). These residual dislocations cannot be avoided by minimising the thermal stress during the crystal growth process.
Investigation of residual dislocations in VGF-grown Si-doped GaAs
Journal of Crystal Growth, 2005
The VGF-method was applied for the growth of Si-doped GaAs crystals of 3 and 4 inch diameter (n410 18 cm À3 ). When the average dislocation density in these crystals falls below 200 cm À2 the stress-induced 601-dislocations disappear and other dislocation types dominate the crystal. These remaining dislocations are classified and the conditions for their occurence are analyzed.
MOVPE growth of AlGaAs/GaInP diode lasers
Journal of Electronic …, 2000
High power diode lasers operating in the wavelength range 730-1060 nm are of significant interest for applications like pumping fiber amplifiers and solid-state lasers, for soldering, material processing or for applications in medicine, spectroscopy and metrology. In recent years Al-free diode lasers have shown improved performance in terms of output power 1 and long-term reliability. 2 Further, the fabrication of buried laser structures is facilitated in the absence of Al. However, the growth of GaInP and InGaAsP alloys on GaAs turns out to be challenging due to difficulties in growing thick ternary and quaternary layers that are precisely lattice-matched and due to miscibility and ordering problems. 3-5 These result in composition fluctuations and rough surfaces. To overcome these problems but to keep most of the advantages of the Al-free system we have combined GaInP waveguide layers with AlGaAs cladding layers for different types of diode laser structures. The formation of abrupt heterointerfaces between AlGaAs and InGaP is crucial for high power devices with good reliability. Several studies have characterized the GaInP/GaAs interface 6,7 and a quaternary intermediate layer between GaInP and GaAs has been re
Effect of Al−content reduction in (AlGa)As cladding layers of MOVPE grown high−power laser diodes
It has been generally accepted to consider AlGaAs/GaAs heterostructures as lattice−matched. Due to the small lattice constant difference between AlAs and GaAs however, the lattice−mismatch in e.g. Al 0.7 Ga 0.3 As/GaAs heterojunction is a/a = 1E−3 [1]. This magnitude is, for instance, only one−fourth of that for doubtless lattice−mismatched GaAs 0.88 P 0.12 /GaAs junction ( a/a = 4.3E−3, tensile−strained GaAsP layer with P−content designed for = 808 nm emission). This means that in heterostructures containing thick Al x Ga 1−x As cladding layers with high x−values one can expect some small perturbations during the epitaxial growth, leading to the QW active region performance deterioration. This is in significant degree dependent on the optimisation of the MOVPE growth conditions, but in general, it would be a good idea to decrease the Al−content in claddings if possible. This would be advantageous not only because of the lattice mismatch decrease, but also because of oxygen and carbon incorporation reduction, dopant activation energy decrease and so on.
Computer-assisted growth of low-EPD GaAs with 3″ diameter by the vertical gradient-freeze technique
Journal of Crystal Growth, 1999
We have grown 3, silicon-doped GaAs crystals with low dislocation density by the vertical gradient freeze (VGF) method. The thermal conditions in a newly designed, multi-zone VGF-furnace were optimized by the aid of numerical simulation. A computer controlled temperature-time program of the 9 heaters was acquired which allows to keep the axial temperature gradient in the solid (liquid) GaAs at the optimized constant values of 7(2) K/cm during the whole growth process. By using these calculated heater temperatures in real growth experiments, we succeeded in growing 3 single crystals with EPD(500 cm\.
Analysis of silicon incorporation into VGF-grown GaAs
Journal of Crystal Growth, 2002
The incorporation of silicon into VGF-grown GaAs is examined by Hall effect measurements, spark source mass spectrometry and photoluminescence (PL). It is found that the silicon is incorporated into the crystal according to Scheils-law with the Si concentration [Si] rising from 1.5 Â 10 18 to 1 Â 10 19 cm À3 . It is found that the intensity of the PL peak with energy close to the band gap decreases with increasing Si content of the material, whereas the intensity of the PL peak related to the acceptor Si Ga V Ga shows opposite behaviour. A compensation model which takes into account the acceptors Si As and Si Ga V Ga is developed. The model describes the relationship between [Si] and the charge carrier concentration n up to silicon concentrations of 1 Â 10 19 cm À3 in GaAs grown under low thermal gradients. r
Metalorganic vapor phase epitaxial growth of GaInAsP/GaAs
Journal of Electronic Materials, 1995
Ga Inl_ASyPl_y lattice matched to GaAs has been grown by low pressure metalorganic phase vapor epitaxy over the entire compositional range. At T~ = 670~ broad peaks of low intensity are observed in the 10K photoluminescence for y = 0.2-0.4 due to the predicted miscibility gap in this compositional region. An increase in growth temperature leads to a smaller miscibility gap. The band gap as well as the morphology show a strong dependence on substrate misorientation. The smoothest GaInAsP surfaces are obtained on exact oriented substrates. For the ternary GaInP the surface roughness is correlated to the degree of ordering in the temperature range of 600 to 750~ The smallest band gap together with the smoothest surface is obtained on (100) 2 ~ off to (lll)B. Ordering effects are also observed in the quaternary GaInAsP. Broad-area lasers processed from the grown layers show high slope efficiency (0.9 W/A) and low internal losses (<3 cm-1).