Low Pressure MOVPE Research Articles

Low-Pressure MOVPE: The time of the pioneers

This paper is the author’s personal view on the development of low-pressure metalorganic vapor phase epitaxy (LP-MOVPE) which took place in the late 70s and early 80s of last century. Pioneering work has been performed by the author and his co-workers in the labs of Thomson CSF, France, leading to many sophisticated devices including GaAs-GaAlAs- and InP-InGaAsP-based quantum well laser diodes which could be driven in CW mode, being grown successfully the first time by LP-MOVPE. This was only possible by a thorough analysis and a deep understanding of the underlying processes, in particular to overcome basic hurdles like the comparably low purity of the available MOVPE precursors at that time. Many results have kept their importance over decades and are still of major significance.

Growth of ZnMgSeTe nearly lattice-matched to ZnTe and p-type doping by low-pressure MOVPE

Zn1-xMgxSeyTe1-y layers have been grown on (100) ZnTe substrates by low-pressure metalorganic vapor phase epitaxy using dimethylzinc, bis-methylcyclopentadienyl magnesium ((MeCp)2Mg), diethylselenide and diethyltelluride as source materials. The Mg mole fraction of Zn1-xMgxSeyTe1-y layer can be controlled successfully by varying (MeCp)2Mg transport rate. P-type doping of this material has been tried using tris-dimethylaminophosphorus as a dopant source. The influence of annealing temperature or dopant transport rate upon the electrical property has been investigated. All the layers show p-type conduction even without annealing. Through this study, a maximum carrier concentration of 2.5×1018cm−3 is obtained.

Photoluminescence and electrical properties of P-doped ZnTe layers grown by low pressure MOVPE

Photoluminescence (PL) spectra and electrical properties of both as-grown and annealed phosphorus-doped ZnTe layers grown by low-pressure metalorganic vapor phase epitaxy have been clarified as a function of reactor pressure or dopant transport rate. The carrier concentration of as-grown layer increases with decreasing reactor pressure or increasing dopant transport rate, in good agreement with PL behavior. Similar tendency is also found for annealed layer. Annealing treatment induces the enhancement of the carrier concentration, independent of reactor pressure or dopant transport rate. Through this study, a high carrier concentration near 1019cm−3 has been attainable.

TEM investigations on growth interrupted samples for the correlation of the dislocation propagation and growth mode variations in AlGaN deposited on SiN x interlayers

Threading dislocations (TDs) can be reduced by in-situ deposition of intermediate SiN x sub-monolayers in group III-nitride heterostructures and their ternary alloys. Here we observe efficient dislocation density decreasing at the SiN x nano-mask in an Al x Ga 1− x N layer with x=0.2 grown epitaxially on c-plane sapphire by low pressure MOVPE. However we did not achieve high annihilation efficiency homogenously along the whole SiN x interface in our samples. Areas with high and low annihilation grade alternate along the interface in accordance with respective variations in the SiN x distribution. Furthermore an unusual dislocation bundling was observed for the dislocations in areas with high dislocation densities above the SiN x interface, leading to large areas of several μm 2 with low dislocation densities at the surface between the dislocation bundles. By using growth interrupted samples under same growth conditions, it was possible to investigate the heterostructures in different growth stages by cross-sectional TEM. This enabled us to correlate the dislocation propagation with growth mode variations in AlGaN deposited on the SiN x interlayer and to develop a growth model for the AlGaN layer grown on the SiN x nano-mask.

AlGaAs/InGaP interfaces in structures prepared by MOVPE

Al 0.3Ga 0.7As/In 1− x Ga x P structures were prepared by low-pressure MOVPE. Lattice matched and strained ones with top In 1− x Ga x P layers as well as reverse ones with top Al 0,3Ga 0,7As layers were examined. The structures were studied by photoluminescence, X-ray and atomic force microscope (AFM) methods. An additional photoluminescence peak from the Al 0.3Ga 0.7As/In 1− x Ga x P interface was observed in our samples and it was attributed to a type-II band offset. A conduction band offset of 0.121 eV was measured in the Al 0.3Ga 0.7As/In 0.485Ga 0.515P lattice-matched structure and a linear dependence of the conduction band offset on In 1− x Ga x P composition, with a zero offset in the Al 0.3Ga 0.7As/In 0.315Ga 0.685P structure, was determined. The valence band discontinuity had a nearly constant value of 0.152 eV.

Growth and studies of Si-doped AlN layers

Edge-type threading dislocations (TDs) can be reduced by increasing the epilayer thickness of undoped AlN grown on c -plane sapphire substrate by low pressure MOVPE. Indium as a surfactant helps to reduce cracks in both, undoped and Si-doped AlN epilayers. For high Si concentrations, edge-type TDs bend and bunch together and finally emerge as V-pits on the epilayer surface. We observed an increasing intensity ratio between the deep level transitions at around 3 eV and the near-band-edge luminescence of the low temperature (10 K) CL spectra for increasing Si concentration, whereas the electrical conductivity decreased in the range of higher doping concentration. A fair electrical conductivity is obtained for a sample having moderate Si concentration of 2 × 10 18 cm - 3 .

Photoluminescence properties of GaSb epitaxial layers passivated in hydrogen plasma

AbstractUndoped and Si‐doped GaSb layers grown by low pressure MOVPE have been treated in hydrogen plasma and the effect of this treatment on photoluminescent (PL) and electrical properties is presented. Hall‐effect and PL measurements reveal that hydrogenation leads to reduction in the concentration of electrically active native acceptors, while the concentration of shallow acceptor SiSb remain practically unaffected. The hydrogenation of layers grown at Sb rich conditions results in nearly complete quenching of the PL line peaking at 896.5 meV, denoted in the literature as BE4. The origin of the BE4 line is ascribed to radiative recombination of excitons bound to a “bare” gallium vacancy. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Aluminum incorporation in [formula omitted]-layers and implications for growth optimization

Al x Ga 1 - x N -layers are applied in various devices based on group-III-nitride semiconductors. Crystalline quality, composition and morphology need to be controlled by properly adjusting growth processes. Here we report on Al x Ga 1 - x N -growth by low pressure MOVPE on planar surfaces of sapphire with focus on Al-incorporation. The solid Al-composition x Al depends critically on growth parameters for constant gas phase compositions. The strongest effect is found for total pressure: Al is efficiently incorporated only at pressures lower than 50 mbar. This can be understood mainly as a result of prereactions in the gas phase involving TMAl and NH 3 which has been discussed earlier in a theoretical paper of the Jensen group [T.G. Mihopoulos, V. Gupta, K.F. Jensen, J. Crystal Growth 195 (1998) 733]. In this publication we compare the predictions of this work with our data and implications for growth optimization.

Controlling emission wavelength of double-capped InAs quantum dots by selective MOVPE employing stripe mask array

The selective area growth by low-pressure MOVPE using a SiO 2 narrow stripe mask array with a wide SiO 2 mask at one side of the array was performed for controlling size and density of quantum dots (QDs). To obtain the QD size difference much further, we have tried the ‘double-cap’ procedure in the selective area growth, and we have successfully controlled the emission from InAs QDs in 75 nm wavelength range on the single chip.

MOVPE growth of GaAs on Ge substrates by inserting a thin low temperature buffer layer

AbstractHigh quality GaAs layers have been grown by low pressure MOVPE on Ge(001) and Ge(001) 9° off oriented in [110] direction by using a thin low temperature (LT) GaAs layer. Investigations of the initial growth step were performed at different V/III ratios and temperatures. To show the good buffer layer quality solar cell structures were grown on off oriented n‐Ge(001) and n‐GaAs(001) substrates. The surface morphology was studied by atomic force microscopy which showed the step‐flow growth mode on 1.2 µm thick GaAs/Ge structures. The crystalline qualities of this structures and the smooth surface morphology were investigated by double crystal X‐ray diffraction (XRD) and atomic force microscopy (AFM). (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Vertically Increasing Well Thickness and In Content in GaInN MQW's due to V-shaped Pits

ABSTRACTIn this work we investigate the influence of V-shaped pits on the growth of GaInN-GaN MQW's by TEM and x-ray diffraction measurements. We have grown different MQW's using low-pressure MOVPE and studied the influence of the number of QW's as well as of the depth of V-shaped pits. During the growth of the samples a material transport takes place from the pits to the c-plane. We show that the thickness of the GaInN QW's and of the GaN barriers is increased due to material transport. With increasing number of quantum wells the diameter of the pits increases, which leads to a super linear increase of these thicknesses. We present a model which describes the change in thickness due to material transport. Another consequence of material transport is a lateral fluctuation in thickness with a characteristic length on the order of the distances between the dislocations.

Recombination dynamics in GaN/AlGaN low dimensional structures obtained by SiH 4 treatment

We present optical properties of GaN/AlGaN structures containing quantum dots, which were obtained in a low-pressure MOVPE reactor on a SiH4-treated surface. Photoluminescence (PL) shows two bright PL bands (P1, P2) and one weak PL band (P3) at energies of 3.48 eV, 3.44 eV and 3.35 eV, respectively. Time-resolved PL shows that the P1, P2 and P3 bands have lifetimes of about 0.05 ns, 0.4 ns and 1.3 ns, respectively. Micro-photoluminescence measurements reveal that these bands consist in fact of many sharp lines coming from small spots on the sample. Such behaviour is characteristic for quantum dots or islands. The P2 shows significant spectral shift with time. As confirmed by power dependent measurements, the spectral shift is due to excitation of high-energy quantum dots followed by diffusion of excitons between QDs to lower energies. Excitation distribution parameters found at low temperature allow explaining of the behaviour of the PL at higher temperatures. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Growth of Al xGa 1−xN-layers on planar and patterned substrates

Al x Ga 1– x N-layers are applied in various devices based on group-III-nitride semiconductors. Crystalline quality, composition and morphology need to be controlled by properly adjusting growth processes. Here, we report on AlGaN growth by low-pressure MOVPE on planar and patterened surfaces of sapphire and 6H-SiC(0 0 0 1) with focus on Al-incorporation and lateral growth. All layers show good surface morphology, near bandedge luminescence and no sign of phase separation (Al-concentration <50%). The Al-incorporation depends critically on growth parameters. For high pressures of 200 mbar Al is hardly incorporated which can be understood as a result of prereactions involving TMA1 and NH 3. For patterened substrates PL indicates lateral variation in Al composition as a consequence of mass transport and a reduced lateral growth rate.

1.5 μm VCSEL structure optimization for high-power and high-temperature operation

MOVPE epitaxial growth and design optimisation of 1.5 μm vertical cavity surface emitting lasers (VCSELs) for high power, single mode and high-temperature operation are presented. The VCSEL structure comprises a strain-compensated InAlGaAs MQW active region, a p ++/n ++ In(Al)GaAs tunnel junction and top and bottom AlGaAs/GaAs fused distributed Bragg reflectors. All epitaxial layers were grown by low-pressure MOVPE in nitrogen atmosphere. Excellent optical and electrical confinement is obtained by wafer-bonding of the top mirror on tunnel junction pre-etched mesas. Devices with 7 μm aperture show single transverse mode CW operation up to 4 mW at room temperature, with 30–35 dB side-mode suppression ratio and a far field divergence angle of 9°. At 85 °C, a record single mode high emission power up to 1.7 mW is demonstrated.

Influence of AlGaN nucleation layers on structural and electrical properties of GaN on 4H-SiC

Using low-pressure MOVPE, with a close-coupled showerhead reactor, Si-doped GaN layers were grown on 4H-SiC substrates. On substrates cut 8° off the (0 0 0 1) plane, Al 0.6Ga 0.4N nucleation layers were grown at 1020 °C with thicknesses ranging from 8 to 250 nm. A thickness of at least 250 nm was required to obtain specular, crack-free GaN layers 3 μm thick. At this thickness the FWHM of the GaN (0 0 0 2) rocking curve was 286 arcsec. A clear relation between AlGaN layer thickness and GaN FWHM exists. However, the GaN surfaces showed undulations with an rms roughness of 8 nm and a peak to valley distance of 40–90 nm. We have investigated lower Al-content layers on exactly oriented 4H-SiC in order to prepare devices with vertical conduction. Si-doped GaN layers were grown on Al 0.3Ga 0.7N and Al 0.08Ga 0.92N nucleation layers deposited at temperatures varying from 800 to 1100 °C. Atomically smooth GaN surfaces were obtained. The lowest FWHM of the GaN (0 0 0 2) rocking curve we achieved using Al 0.08Ga 0.92N was 189 arcsec. The XRD peaks of all layers grown on Al 0.3Ga 0.7N layers were significantly broader, indicating a more defective crystal structure. Remarkably, the measured electron mobilities in these samples were systematically higher than in the GaN layers grown on Al 0.08Ga 0.92N. It appears that the higher mobilities are obtained with higher compressive stress in the GaN layer.

Electroluminescence from GalnN Quantum Wells Grown on Non-(0001) Facets of Selectively Grown GaN Stripes

ABSTRACTNon (0001) GalnN QWs have been grown by low pressure MOVPE on side facets of triangular shaped selectively grown GaN stripes. By analysing low temperature photo- and cathodoluminescence and room temperature electroluminescence, we found strong indications, that both, In and Mg are less efficiently incorporated on these side facets compared to the common (0001) plane with even lower efficiency for stripes running along (1–100) compared to (11–20). Nevertheless, we observed strong light emission from these quantum wells, supposed to be at least partly caused by the reduced piezo-electric field.

Reduction of dislocation density in AlGaN with high AlN molar fraction by using a rugged AlN epilayer

ABSTRACTA method for making AlGaN with a high AlN molar fraction and low dislocation density is needed for fabricating deep ultraviolet emitters and detectors. In this study, we reduced the dislocation density in AlGaN over a large surface area by using low-pressure MOVPE on a continuously rugged epitaxial AlN substrate. The AlN molar fraction of the AlxGa1-xN was × = 0.51, and atomic steps in the surface were clearly observed with an atomic force microscope (AFM). In addition, the dislocation density was estimated to be 8.8 × 107 cm−2, which is two orders of magnitude lower than that of AlGaN grown on a flat AlN epitaxial layer. Our results indicate that the dislocation density of AlGaN can be greatly reduced by using a rugged AlN epitaxial substrate with continuously inclined facet.

Growth of Al x Ga 1− x N and GaN on photo‐electrochemically patterned SiC substrates

The large defect densities in heteroepitaxially grown group-III-nitride layers on sapphire or SiC cannot be tolerated in applications such as lasers. As an alternative to the well known ELOG process for defect reduction we report here on overgrowth of patterned 6H–SiC(0001) surfaces. First, we formed mesa structures in the windows of metal masks and then after removal of the masks layers of AlxGa1−xN and GaN were grown by low-pressure MOVPE under conditions of high lateral growth rates. We demonstrate that layers and layered structures can be grown with smooth surfaces and reduced defect densities (a factor 5–10 lower compared to growth on planar substrates). For a further reduction of defect densities a detailed understanding of the growth process on patterned substrates is necessary. Growth starts at the sidewalls of the mesa. For a defect reduction low pressures and high V/III-ratios are required. Very high lateral-to-vertical growth rates of more than 8 can be achieved on such structures. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Optimization of GaN/AlGaN Quantum Wells for Ultraviolet Emitters

ABSTRACTWe have studied GaN/AlGaN and GaInN/GaN quantum well structures grown on sapphire and SiC substrates using low-pressure MOVPE. The emission wavelength of the GaN/AlGaN QW's was varied in the 320–400 nm range and that of the GaInN/GaN QW's in the 390–480 nm range, by adjusting the QW width. Using temperature dependent resonant-excitation photoluminescence measurements we have determined the internal quantum efficiency (IQE) and analyzed its temperature dependence. For the GaN/AlGaN QW's we achieve a reasonable IQE of in excess of 5 % only at fairly high excitation power, while GaInN/GaN QW's provide IQE's of up to 46 % at rather low power density, indicating a much stronger role of defects for the former. While thermal activation energies are dominated by intrinsic mechanisms for GaInN/GaN QW's, lower activation barriers evident from the GaN/AlGaN data also hint at a more import role of defects.

Structural characterization of ZnSe/ZnMgSe MQWs grown on (1 0 0)GaAs by low pressure MOVPE

A detailed structural characterization of ZnSe/ZnMgSe multiple quantum wells (MQWs) grown on GaAs by low pressure metalorganic vapour phase epitaxy is presented. ZnSe/Zn 0.83Mg 0.17Se MQWs having between 6 and 12 periods were deposited at 330°C and 304 mbar reactor pressure on (1 0 0)GaAs after a 4.2 nm ZnSe buffer layer. The MQWs had nominal 4.4 nm thick ZnSe wells and 5.3 nm thick Zn 0.83Mg 0.17Se barriers. The MQW structural properties were investigated by high-resolution X-ray diffraction (HRXRD) and X-ray specular reflectivity (XSR) measurements. Besides the MQWs-substrate mismatch, simulation of the HRXRD and XSR patterns allowed to determine the MQW period, individual layer thickness and barrier composition. Between 8 and 10 periods the MQW structure begins to relax, its critical thickness on GaAs being between 92 and 113 nm. Furthermore, HRXRD showed broader zeroth and first-order satellite peaks with increasing MQW periods, a result ascribed to strain fluctuations induced by either inhomogeneous Mg incorporation in the ZnSe lattice and/or interface roughening. Comparison of experimental and simulated XSR patterns allowed to determine the rms roughness at each multilayer interface, which linearly increases along the growth direction due to a cumulative intrinsic roughening.