We have examined various methods of overcoming the problems of low growth efficiency and high carbon uptake which occur when GaAs is grown at low temperatures by Metal Organic Molecular Beam Epitaxy (MOMBE). We have found that removal of the carbon through conventional means such as precracking or interaction with hydrogen is not effective in enhancing the carbon removal process. In fact, the use of a hydrogen plasma during growth actually increases the carbon background due to a reduction in the surface V/III ratio. Greater success is obtained when alternative precursors are used as replacements for AsH3 and triethylgallium (TEG). Tris-dimethylaminoarsenic (DMAAs) offers reduced carbon uptake through formation of amine compounds while tri-isobutylgallium (TTBG) shows better efficiency and less carbon than TEG at low growth temperature.

The fabrication of Photonic Integrated Circuits (PIC) requires the development of advanced growth and processing techniques. One of the major problems in the fabrication of PICs is the monolithic integration of passive and active waveguiding structures with a different bandgap. This is schematically shown in figure 1 where a laser, waveguide and detector are integrated on the same substrate. The following relationship between the different bandgaps is required : Eg (detector) < Eg (laser) < Eg (waveguide). One of the most advanced PICs is certainly a coherent receiver chip where a local DFB laser oscillator is integrated with a Y-junction, 3-dB splitter and balanced photodetector pair [1,2,3]. Current integration schemes are mostly based on the use of different epitaxial growth steps to obtain the different bandgap materials on the same substrate. In order to improve yield and performance it is required to reduce the number of growth steps by using special growth techniques. In this paper we will briefly describe some of the recent developments in advanced growth techniques. A more detailed description will be given of our recent work based on selective growth and shadow masked growth using Metal Organic Vapour Phase Epitaxy (MOVPE).

In the growth of GaAs and GaP from TMG strong differences are observed. Although the shape of the Arrhenius plot of the growth rate is similar, at low deposition temperatures the GaP growth rate is lower than that of GaAs. Additionally, more carbon is incorporated into GaP than into GaAs. Mass spectrometric studies on methyl desorption show that As has a stronger ability to aid the breaking of the final Ga-carbon bond than P.

We report on the use of thermally-cracked tertiarybutylphosphine (TBP) and tertiarybutylarsine (TBA) with elemental Ga, In, and Al sources for the MOMBE growth of InP-based resonant tunneling diode (RTD) and resonant tunneling bipolar transistor (RTBT) structures. We have systematically examined the effects of growth conditions and heterostructure modifications on the InP/lnGaAs RTD including the use of pseudomorphic (InGa)P barriers and, in addition, explored for the first time, InP quantum well RTDs using both AlAs and InGaP barriers. Cross-sectional transmission electron microscopy has been used to correlate the structural quality with the electrical characteristics for both lattice-matched and pseudomorphic layers composed of InAs, AlAs, and InGaP. We also demonstrate the first use of mixed InP/lnGaAs and AlAs/lnGaAs heterojunctions in the RTBT. These transistors exhibit room temperature negative transconductance and a peak-to-valley current ratio of 35, the highest yet observed In the RTBT.

Dislocation formation in InAs1−xSbx buffer layers grown on InSb substrates by metal-organic chemical vapor deposition is shown to be reproducibly enhanced by p-type doping at levels exceeding the intrinsic carrier concentration at the growth temperature. To achieve a carrier concentration greater than 2 × 1018 cm−3, the intrinsic carrier concentration of InSb at 475 C, p-type doping with diethylzinc was used. Carrier concentrations up to 6 × 1018 cm−3 were obtained. The zinc doped buffer layers have proven to be reproducibly crack free for InAs1−xSbx step graded buffer layers with a final composition of x = 0.12 lattice matched to a strained layer superlattice (SLS) with an average composition of x = 0.09. These structures have been used to prepare infrared photodiodes. Details of the buffer layer growth, an explanation for the observed Fermi level effect and the growth and characterization of an infrared photodiode are discussed.

The carbon doping properties of GaAs with carbon tetrachloride as the dopant source were examined using trimethylgallium (TMGa) or triethylgallium (TEGa) as the gallium precursors and arsine or tertiarybutylarsine (TBAs) as the arsenic precursors. Secondary ion mass spectrometry (SIMS) and Hall measurements (van der Pauw method) were used to characterize the epitaxial GaAs:C layers. Very high C-doping concentrations (∼1020 cm−3) could be obtained with either TMGa and TEGa. The use of TBAs instead of AsH3 led to a significant reduction in carbon incorporation, by approximately a factor of 5–10 per mole of As precursor, over the temperature range examined (520°C - 700°C). Hydrogen at significant concentrations (0.5 – 6 × 1019 cm−3) was detected by SIMS in GaAs:C layers grown at ≤550°C utilizing all four combinations of Ga/As precursors and suggested the presence of electrically inactive C-H complexes. A post-growth anneal under helium at 550°C for 60s of these samples resulted in a 50–100% increase in hole concentration by driving out the hydrogen.

Carbon is a promising p-type dopant in GaAs/AlxGa1−xAs heterojunction bipolar transistors (HBT) because of its low atomic mobility and its potential for achieving very high carrier concentrations. It is generally believed that carbon incorporates substitutionally on the column V sublattice. However, an anomalous behavior at carrier concentrations > 5 × 1019 cm−3 is observed in the electrical properties of carbon doped layers. The strain sustained in these layers may be explained by the presence of interstitial carbon.

We used Rutherford Backscattering Spectrometry in channeling geometry utilizing the nuclear reaction 12C (d,p)13C to determine the lattice locations of carbon in GaAs. The data presented unambiguously show, that up to 25% of the carbon atoms occupy interstitial sites. The presence of interstitial carbon is of importance for applications, since interstitial carbon may exhibit an enhanced diffusivity altering nominally abrupt dopant profiles.

Because of the extreme toxicity ofA3, it is highly desirable to employ gaseous As sources which contain fewer As-H bonds. Attempts to introduce compounds such as tertiarybutylarsine (TBAs) during growth by metal-organic molecular beam epitaxy (MOMBE) have been somewhat unsuccessful due to the need for pre-cracking of these materials, and to the extreme reactivity of the hydrocarbon radicals released upon their decomposition. These byproducts have been found to severely degrade various components in the growth system, and could lead to enhanced carbon uptake at low growth temperatures. Phenylarsine (PhAs) offers several advantages over the more common As substitutes as it has been demonstrated to decompose at growth temperatures of ≥575°C, and the byproducts of its decomposition are expected to be far less reactive than the byproducts of the other As precursors.

In this paper we will discuss the growth of GaAs and AlGaAs at low growth temperatures (≤530°C) using PhAs as the As source. In this temperature range, the III-V growth rate is restricted due to the cracking efficiency of the PhAs. For example, at 530°C, a PhAs flow rate of ∼5.4 seem limits the growth rate to ∼95 Å/ min while a similar flow of AsH3 through a low pressure cracker allows for deposition at rates >250 Å/min. Further comparisons of the two As sources will be discussed regarding their effect on GaAs and AlGaAs growth rates from triethylgallium, trimethylgallium, and trimethylamine alane, and their effect on carbon and oxygen impurity incorporation.

We have investigated the effect of growth temperature and V/III ratio on dopant incorporation from disilane (Si2H6) and tetraethyltin (TESn) over the temperature range 475°C-525°C during growth of GaAs by metal organic molecular beam epitaxy (MOMBE). Increasing V/III ratio produced only a slight decrease in the dopant concentration while increasing growth temperature resulted in slighdy higher dopant levels. Addition of Al and H to the growth surface via introduction of trimethylamine alane had no apparent effect on dopant incorporation. No significant differences were observed in the incorporation behaviors of Si2H6 and TESn, and both sources yielded comparable base-collector junction behavior when used for growth of heterojunction bipolar transistors (HBTs).

The use of trimethylamine alane (TMAAl) as an alternative to trimethylaluminum (TMAl) for low-pressure metalorganic vapor-phase epitaxy (MOVPE) of AlGaAs thin films as well as complex optoelectronic device structures has been studied in detail. AlGaAs layers were grown in a horizontal reaction chamber at 20–110 mbar with growth temperatures in the range 650°C≤TG≤750°C. Wafer thickness uniformity is strongly dependent on growth pressure, and is acceptable only for the highest linear flow velocities. The 12K photoluminescence (PL) spectra of AlGaAs layers grown using TMAAl and TEGa exhibit uniformly intense and narrow bound-exciton emission throughout the growth temperature range investigated. To assess the viability of this new source for the low-pressure OMVPE growth of advanced optoelectronic devices, several optically-pumped vertical-cavity surface-emitting laser (VCSEL) structures were grown using TMAAl extensively. Room temperature lasing at 850 nm was reproducibly obtained from the VCSEL structures, with a threshold pumping power comparable to similar structures grown by molecular beam epitaxy in our laboratories.

Carbon is readily incorporated into epitaxial GaAs and related alloys grown from metalorganic sources. Hydrogen is also readily incorporated during growth and processing from essentially every possible source including the metalorganics, AsH3, and H2 that are used during growth or in annealing ambients. This hydrogen forms stable neutral complexes with carbon thereby altering the intended p-type doping. In this paper, the properties of the C-H complexes as well as the sources of hydrogen, the stability of passivation, and the concentration of C-H complexes in epitaxial layers will be discussed.

Optical measurements have been performed on heavily carbon doped GaAs layers grown on semi-insulating GaAs substrates by MOMBE (metal-organic molecule beam epitaxy). Photoluminescence excitation (PLE) spectroscopy was used to measure the onsets of optical absorption in these GaAs:C epilayers. It was found that in samples with free carrier concentrations of 6.2×1019, 1.6×1020, and 4.1×1020cm−3, optical absorption begins at 1.40, 1.52, and 1.53 ev, respectively. Combined with the band gap narrowing data from photoluminescence (PL) spectra, we estimated Fermi level locations relative to the top of the valence band. We also measured reflectance in the near infrared region and estimated the effective mass of free holes using a classical two-oscillator model.

Photoluminescence spectra of AlGaAs/GaAs heterostructures with layer thicknesses in the micrometer range show excitonic recombination peaks from the AlGaAs as well as the GaAs layer. Luminescence in the buried GaAs layer may be produced by charge carrier diffusion across the interface and/or photon recycling. We have monitored the luminescence intensity from both layers as a function of laser power in order to determine the dominant generation process in the GaAs layer. The ambi polar diffusion equation has been solved to derive the charge carrier distribution. Based on these data the relative intensities of the AlGaAs and the GaAs excitonic luminescence can be used to obtain information about the interface recombination in a nondestructive way. This characterization method has been applied to investigate the quality of GaAsP/GaAs interfaces as a function of increasing lattice mismatch and dislocation density.

We have developed a novel experimental technique for accurately determining band offsets in semiconductor quantum wells (QW). It is based on the fact that the ground state heavy- hole (HH) band energy is more sensitive to the depth of the valence band well than the light-hole (LH) band energy. Further, it is well known that as a function of the well width, Lz, the energy difference between the LH and HH excitons in a lattice matched, unstrained QW system experiences a maximum. Calculations show that the position, and more importantly, the magnitude of this maximum is a sensitive function of the valence band offset, Qy, which determines the depth of the valence band well. By fitting experimentally measured LH-HH splittings as a function of Lz, an accurate determination of band offsets can be derived. We further reduce the experimental uncertainty by plotting LH-HH as a function of HH energy (which is a function of Lz ) rather than Lz itself, since then all of the relevant parameters can be precisely determined from absorption spectroscopy alone. Using this technique, we have derived the conduction band offsets for several material systems and, where a consensus has developed, have obtained values in good agreement with other determinations.

Resonance ionization mass spectrometry (RIMS) of neutral atoms sputtered from III-V heterostructure semiconductor materials provides quantitative information about the dopant position near interfaces. The prerequisite for quantitative results is the saturation of the ionization step. The absolute signals are affected by primary ion beam parameters which affect sputter yield, atomization efficiency and quantum state partitioning, but not ionization efficiency. We have found that matrix effects are minimal and use RIMS results to help elucidate dopant migration near interfaces and interpret SIMS matrix effects. Device performance and understanding of materials growth are both aided.

Transmission electron microscopy studies have been performed to characterise InxAl1−xAS layers grown by Molecular Beam Epitaxy on (100) InP substrates. The first observations of compositional nonuniformities in strained InAlAs layers are reported. The coarse quasiperiodic structure present in each sample has been found to be dependent upon the growth parameters and the sample characteristics such as strain, thickness and x value.

The acquisition of RHEED oscillation information on (100) GaAs substrates is described for use in the growth of “lattice-matched] Iny (AlxGa1−x)1-Y As layers on (100) InP substrates with 0.52 < y < 0.53 and 0.00 < x < 1.00. The observed frequency of the RHEED oscillations on GaAs is the same as on InP, however, the measured lattice parameters of the grown layers are less than that of InP. The x-ray diffraction images show that the misfit dislocations perpendicular to the primary flats of 2” round (100) InP wafers are denser than the parallel ones. Photoluminescence (at 10K) and photoreflectance (at 300K) measurements on a composite layer structure of x=0, 0.2, 0.4, 0.6, 0.8 and 1 clearly show six distinct peaks with narrow FWHMs of less than 20 meV. The measured bandgaps increase linearly with the Al content.

Threading dislocation morphologies and characters, as well as their generation conditions in InxGa1−xAs films grown by molecular-beam epitaxy on GaAs (001) substrates have been examined, mainly using cross-section al transmission electron microscopy (XTEM) as a function of x and film thickness. The formation of severe threading dislocations is detected in epilayers ofx≧0.2 at a fixed film thickness of 3 μm and with film thicknesses greater than 2μmat x=0.2. Most of the observed threading dislocations are 60°- and pure-edge type dislocations along the <211> and [001] directions, respectively. The former type dislocations are mainly observed in layers of x≧0.2; the latter predominantly exist in layers of X≧O.3.

We present a new version of a deep level transient spectroscopy which is suitable for non-contact, non-destructive determination of deep level defects in semiconductor wafers without preparation of metal-semiconductor diodes or p-n junctions.

The method relies on deep level thermal emission measurements by the surface photovoltage (SPV) transient following an optical filling pulse. Non-equilibrium occupation of deep levels is realized within the native surface depletion region by the capture of excess minority carriers. Since the native Schottky-type surface barrier is commonly present on semiconductor surfaces, the approach requires no wafer pre-treatments. Non-contact SPV measurements are realized using a capacitive coupling to the wafer front and the wafer back.

The quantitative principles of the SPV-DLTS approach are discussed using experimental data obtained on GaAs.

Many advanced III - V devices require highly strained heteroepitaxial layers less than 25 nm in thickness, with tight specifications on both the layer thickness and composition. In many cases the layers required are close to the critical thickness.

The growth conditions for these thin layers are often extrapolated from established conditions for thicker layers. This method can result in layers which have the incorrect thickness and composition because of the transients which occur as growth commences. To minimise this problem it is desirable to establish growth conditions for layers which are as close to device requirements as possible. X-ray diffraction is capable of measuring layer thicknesses and compositions non-destructively. The minimum measurable layer thickness is usually within a small factor (typically 0.5 to 5 times) of device requirements.

A single x-ray rocking curve is required to determine the thickness and composition of an unrelaxed (strained) layer. At least two rocking curves are required when relaxation is present. This paper discusses the appropriate choice of measurement conditions for a given sample.