The overall objective of the CONFORM project was to determine precisely the potentialities and to develop, in parallel by MOVPE and HVPE, the conformal growth technique, known to produce high quality GaAs layers on Si. The final expected demonstrators were lateral LEDs with various structures, as advanced as possible (double heterostructure with vertical quantum wells ideally).
Intermediate objectives were
1) the demonstration of conformal growth by MOVPE what required the development and selection of suitable precursors and the optimisation of the growth conditions,
2) the mastering of the selective and conformal growth processes by HVPE and
3) the successful modulation of both doping (n- and p-type) and composition during conformal growth by both techniques.
Precursors for selective and conformal growths
The development of the optimum precursor systems for the selective and conformal growths of In, Ga and Al alloys by MOVPE was first carried out. Chemical compatibility and source purity were key topics to establish selective deposition capabilities and to enable device quality structures to be fabricated. Epichem were expected to deliver novel precursors of the highest possible purity to the growth team and establish the best combinations. Also strategies were to be prepared to enable the introduction of the optimum precursor combinations to a commercial production environment.
Epichem have been successful in identifying the optimum precursors for conformal MOVPE of In, Ga and Al alloys and establish production methods capable of yielding ultra high purity batches of these materials. Supply of source material to the growth team has been maintained throughout the project.
The precursor development performed by Epichem was intended to result in a novel range of precursors suitable for the conformal growth technique. Progress from this project has allowed the identification of Me2MCl derivatives as particularly useful and these products will be made available commercially to customers in this market area.
Selective and conformal growths by MOVPE
The study of the selective growth using the different precursor combinations showed the best results for the precursors DMGaCl and DMAlCl. Using standard precursors with additional HCl, no good selectivity together with high growth rates could be achieved. The precursors DTBGaCl and DTBAlCl showed a vapour pressure, too low for obtaining high growth rates for using these sources at room temperature. Using the precursor combinations DEGaCl and DEAlCl as well as DMGaCl and DMAlCl resulted in both cases in selective growth. However, for the precursors DMGaCl and DMAlCl selectivity was the best and higher growth rates could be achieved. So these precursors were selected for the conformal growth experiments.
The influence of the growth parameters and the seeds on conformal growth by MOVPE was studied in detail. The importance of the seeds was demonstrated: only for using seeds with a sufficiently high quality, the deposition of defect-free GaAs on Si with a smooth growth front was possible by MOVPE.
The conformal epitaxy of AlGaAs was also investigated. The conformal growth of AlGaAs has been mastered. However, it was not possible to achieve a smooth formation of the growth front.
The conformal deposition of InGaAs was not possible. The vapour pressure of the precursor DMInCl was too low for obtaining suitable growth rates. Using the standard precursors TMIn pre-reactions occurred, leading to polycrystalline nucleations.
The modulation of composition was investigated. The deposition of GaAs-AlGaAs and AlGaAs-GaAs heterostructures was successfully studied. However, only the GaAs-AlGaAs heterostructures have a sharp interface. The fabrication of conformal AlGaAs-GaAs heterostructures or double heterostructures having a sharp interface is not possible, since it is not possible to deposit AlGaAs having a smooth growth front.
Also the doping and modulation of doping was successfully investigated. For the n-type doping, H2S turned out being a suitable precursor for doping. p-type doping was studied using DEZn and CBr4. Both precursors were suitable for the doping of unpatterned substrates. However, using DEZn polycrystalline depositions occurred during conformal growth. Using CBr4, the doping of conformal structures was successful.
Selective and conformal growths by HVPE
LASMEA objectives were to determine in close interaction with THOMSON-CSF/LCR:
the potentiality of the HVPE (Hydride Vapour Phase Epitaxy) growth technique for selective and conformal epitaxy applications and
to assess the feasibility of laterally modulated GaAs-related structures by conformal HVPE.
The HVPE technique was expected to yield easy selective epitaxy conditions resulting in a complete freedom for the choice of the growth parameters (composition of the vapour phase and temperature) and of the mask features. HVPE growth is known to be limited by surface kinetics that is the kinetics of adsorption and decomposition of the growth precursors on the substrate surface. As a consequence, HVPE was supposed, if mastered, to be a powerful tool for the control of the growth morphologies.
As a result, LASMEA have been successful in stabilising various reproducible morphological profiles by selective growth as a function of experimental parameters of which influence was clearly determined. This control was efficiently exploited for the making of extended low defect GaAs films on Si by conformal HVPE, to be used as high quality GaAs/Si pseudo-substrates for further vertical re-growth of active GaAs devices.
The feasibility of laterally modulated structures by conformal HVPE was demonstrated with the achievement of good quality lateral n/p GaAs homojunctions on Si. Abrupt transitions between the n and p layers were obtained by stabilising mono-facetted lateral growth fronts. LASMEA has clearly demonstrated that lateral selective doping could be achieved on a sub-micrometer scale by means of the conformal growth technique carried out by HVPE. Lateral heterojunctions involving GaInAs quantum wells were grown, emphasizing the potentialities of HVPE for the making of lateral modulated structures. Good crystalline quality and electro-optical properties were demonstrated by THOMSON-CSF/LCR on these structures.
Characterisations of layers and structures
Accurate characterisations of the material properties have been obtained with a high spatial resolution at the University of Valladolid, thanks to the µ-PL, cathodoluminescence and µ-Raman techniques mainly. These precious results have represented key inputs for the growth conditions and structures optimisation. In addition a precise understanding of the physical properties of the conformal layers, particularly of their state of stress, was obtained.
Characterisations of lateral structures have shown promising results paving the way for future development of new light emitting devices.
In summary, conformal growth was demonstrated for the first time by MOVPE, thanks to the development of suitable precursors and optimisation of the growth conditions, but only for simple GaAs/AlGaAs lateral structures, what makes further exploitation difficult. At the contrary, the promising results obtained up to now by HVPE enable to envisage potential applications:
- First conformal layers with an extension in the range of 30-40µm are available and are suitable for integration on silicon of active GaAs devices by vertical regrowth (conformal layers used as high quality pseudo-substrates). The compatibility of this integration procedure with classical (0.8µm) and advanced (0.18µm) CMOS technology was investigated and nearly fully demonstrated in the frame of an other European Project (Esprit MEL-ARI MONOLITH).
- Then the demonstration of an accurate control of the growing facets, together with the first successful experiments on modulation make it realistic to produce lateral devices by this technique. Nevertheless further developments are needed to realise complex ternary or quaternary-based lateral devices. The acquired know-how and mastering of the growth process pave also the way to other applications, like the realisation of microstructures (photonic bandgap, periodically-poled GaAs...) particularly interesting for the optoelectronic or non-linear optic fields.
In spite of its crucial interest for such applications as intra chip optical communications or power devices, heteroepitaxial GaAs on Si has been, to date, largely unsuccessful because the GaAs material quality has never allowed the fabrication of optoelectronic devices exhibiting performances and reliability levels compatible with industrial requirements. Since about six years, a novel technique of confined epitaxial growth (conformal epitaxy), capable of yielding low dislocation density GaAs films on Si, has been developed. So far, chloride vapour phase epitaxy (VPE) has been used to implement this conformal growth, and defect-free layers (defect densities lower than 105cm~2) have effectively been obtained over lateral extensions of about 50Rm. Recently, conformal heteroepitaxial growth of high quality InP layers on GaAs seeds (on Silicon substrates) has also been demonstrated using Hydride Vapor Phase Epitaxy (HVPE). Our goal now is to prove the efficiency of this conformal technique in integrating monolithically high quality optoelectronic devices on Silicon.
Two types of components will be envisaged:
1) standard vertical structures could be grown and processed on conformal layers used as substrates;
2) lateral structures could be obtained by modulating composition and doping during conformal growth.
So, original planar components, unrealisable by other growth techniques, with new potentialities and improved performances could be produced. Two complementary growth techniques will be used to develop these components. First HVPE, having already demonstrating high conformal growth rates and controlled facing of growth front appears very suitable for producing wide conformal layers and studying lateral modulations. But this technique presents some drawbacks (no Al-compounds, low sharpness of interfaces), which could limit its ability to produce a broad variety of devices with optimum performances. In order to fully exploit the potentialities of conformal growth, we will also develop this technique using MOVPE, which could produce accurate vertical and lateral epitaxy of complex and various structures (but likely with low growth rates hindering the production of wide conformal layers). We choose as first demonstrators vertical double heterostructure GaAIAs/GaAs LEDs grown from GaAs seeds on Si and lateral GaAlAs/GaAs and GaInAsP/GaInP LEDs.
This goal will be reached through different steps, the major ones being:
1) feasibility of selective epitaxy and conformal growth on silicon of undoped GaAs by HVPE and GaAlAs by MOVPE: optoelectronic quality will be required for the material at this step;
2) modulation of doping and composition during conformal growth: accurate control and homogeneity of the n- and p-type doping levels, sharpness of the interfaces between differently doped or composed regions will be critical points of this step;
3) Processing of the epilayers for the fabrication of LED demonstrators.
This consortium brings together three University partners for fundamental studies on the growth mechanisms and for fine characterisations of the obtained material properties and two industrial partners for the industrial development of the growth technique and for the development of specific precursors for selective and conformal growths and doping. The industrial development of this new technique will represent an important breakthrough in semiconductor devices field, because it will improve some present technologies (like high-power laser diodes processing) and enable the development of totally new performant components (like lateral LEDs) or systems (e.g. optically interconnected Si IC's). The extension of this technique to other heteroepitaxial systems like InP on Si or m-v nitrides on sapphire will also become possible and attractive BE97-4071.
Funding SchemeCSC - Cost-sharing contracts