Search

Transmission electron microscopy (TEM) is carried out to characterize the extended defect reduction in low-defect nonplanar GaN substrate templates grown by lateral epitaxial overgrowth (LEO). The LEO nonplanar GaN substrate template has a trapezoidal cross section with smooth (0 0 0 1) and { 1 1 2 ¯ 2 } facets. We demonstrate here the dislocation distribution and behavior in both ordinary LEO and two-step LEO. Penetration of threading dislocations (TDs) beyond mask windows is observed in ordinary LEO substrates. In two-step LEO substrates, which utilize the tendency for TDs to bend 90° at certain plane interfaces, only a type dislocations with Burgers vector b = 1 3 〈 1 1 2 ¯ 0 〉 are generated in the upper part above the TD bending zone between two mask windows with a density of ∼8×10 7 cm −2 , and there are almost no dislocations in the LEO wing region. This approach provides a promising path to produce low-defect GaN substrate templates for high-performance buried heterostructure lasers.

Transmission electron microscopy techniques are applied to investigate three-dimensional (3D) microstructures of the GaN nonplanar substrate selectively grown by metalorganic chemical vapor deposition. Two-step lateral epitaxial overgrowth (LEO) has been utilized and optimized to fabricate fully coalesced nonplanar mesa substrate templates with the trapezoidal cross-section. All threading dislocations (TDs) penetrating beyond the two adjacent mask windows are engineered to bend 90° in the lower TD bending layer after the first step of growth. The dislocations, which approach the GaN mesa top, are predominantly perfect a type dislocations with Burgers vectors of 1 3 〈 1 1 2 ¯ 0 〉 and a density of 8×10 7 cm −2 , which is reduced by three orders of magnitude compared with that of bulk GaN. The spatial distribution of different types of dislocations in the LEO nonplanar substrate is demonstrated herein. The main sources of a type dislocations in the post-bending layer are byproducts of dislocation reactions occurring at the TD bending layer.

Defect formation and In segregation in InGaN/GaN quantum wells grown at various temperatures and with various well thicknesses were studied using cathodoluminescence, transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HR-TEM). In 0.16 Ga 0.84 N quantum wells with In compositional nonuniformity and a small number of structural defects originating at the InGaN/GaN interface showed sharp and intense InGaN emission in cathodoluminescence (CL). Increasing the In composition caused surface roughness in the InGaN layer and the formation of stacking faults/dislocations originating at InGaN/GaN interface. This resulted in a reduction of the InGaN emission peak intensity. ( 1998 Elsevier Science B.V. All rights reserved.

Triethylgallium is used in combination with arsine in selective area deposition of GaAs by laser-assisted atomic layer epitaxy with the 514.5 nm line of an Ar ion laser. In addition to the much lower laser intensity required to achieve monolayer self-limiting growth than that using trimethylgallium, an intense room temperature photoluminescence response is observable from the double heterostructures of Al 0.3 Gaa 0.7 As/GaAs with the central GaAs grown by this technique, indicating good quality of the GaAs material and interfaces. The GaAs also exhibits low C contamination levels as is evidenced by capacitance—voltage and secondary ion mass spectrometry measurements. GaAs and Zn-doped p + -GaAs grown by laser-assisted atomic layer epitaxy are incorporated in broad area laser devices for the first time. A threshold current density as low as 544 A cm −2 is obtained on a 570 μm long device under pulsed testing conditions at a 10 kHz repetition rate.

An in situ real-time study of surface reactions in atomic layer epitaxy (ALE) of GaAs using trimethylgallium (TMGa) by reflectance difference spectroscopy (RDS) and sampled beam mass spectroscopy is reported. These studies reveal several phases in atomic layer epitaxy of GaAs and the self-limiting atomic layer deposition of epilayers using TMGa. The results also lend further support to a proposed reaction kinetics model from a recent RD investigation. The self-limiting mechanism which occurs during TMGa exposure cycles is believed to result from both selective adsorption and reaction of TMGa at As atoms and Ga-vacancy-induced Ga-rich surface reconstructions. It is also shown that strategies for optimal ALE growth of GaAs can be obtained through RD monitoring.

The issues involved in the growth of strained InGaAs/GaAs quantum well lasers integrated with AlAs/GaAs Bragg reflectors on nonplanar substrates is treated in this work. Under optimized conditions, controlled growth of uniform small area Bragg reflectors integrated with low threshold lasers has been achieved. An average pulsed threshold current of 2.9 mA/μm for edge emitting lasers at 0.98 μm with 90% external efficiency was obtained showing the potential for the use of this structures as 45° folded cavity surface emitting lasers.

Metalorganic chemical vapor deposition (MOCVD) has been used to grow low threshold 1.3 μm lattice matched and strained quantum well lasers. By careful optimization of growth conditions and growth interruptions between different layers, we have been able to grow good quality lattice matched and strained In x Ga 1- x As y P 1- y layers and quantum wells. This is demonstrated by achieving broad area lasers with threshold current densities as low as 190 A/cm 2 for a two quantum well device with 0.85% compressive strain.

Adsorption of TMGa on thin overlayers of GaAs/AlAs and InAs/InP structures is investigated. The number of Ga monolayers is determined precisely from such structures for various TMGa exposures. Slower reaction rates for TMGa on Ga-stabilized surfaces compared to As-stabilized surfaces result in the self limiting mechanism of atomic layer epitaxy (ALE) using TMGa. On the other hand, adsorption of TMGa on InAs (001) surfaces involves more than the very top layer atoms. Exchange of In/Ga is observed with In atoms floating on top of the surface, which enhances Ga atom formation and impedes saturation as in the case of GaAs surfaces.

Atomic layer epitaxy (ALE) is a relatively new growth technology for depositing compound semiconductors one monolayer at a time. By employing a new regime of metalorganic chemical vapor deposition (MOCVD) growth, in which saturated surface reactions control the growth, it is possible to alternately deposit monolayers of column III and column V elements so that only one monolayer of the III–V compound semiconductor is formed in every cycle of the deposition. The use of metalorganic precursors for ALE is of considerable importance since it allows the hybridization of ALE with the existing MOCVD technique. Several benefits can be realized by integrating the two technologies. Layers of critical thickness and uniformity requirements can be grown by ALE, while thicker epitaxial layers can be grown by MOCVD. Additional advantages are the “digital growth” nature of ALE which affords a high degree of thickness reproducibility, and the selective area growth potential of laser-assisted ALE (LALE). In this paper, ALE and LALE of GaAs is reviewed with an emphasis on the utilization of metalorganic precursors.

Atomic layer epitaxy (ALE) is a regime of metalorganic vapor phase epitaxial growth in which uniform growth of ultra-thin epitaxial layers by a self-limiting monolayer by monolayer deposition process is achieved. In this paper ALE has been applied to the growth of single crystal GaAs, AlAs and GaAs/AlGaAs heterostructures and devices. Data are presented that show one monolayer uniformity in ultra-thin layers grown by ALE. The dependence of growth rate on reactant flows and temperature are described. Cleaved corner TEM analysis of ALE epitaxial layer thicknesses demonstrates the “digital” nature of the deposition process. The low temperature photoluminescence (PL) of ALE grown GaAs quantum wells exhibit narrow line intrinsic luminescence with linewidths comparable to the best reported values by conventional MOCVD. Quantum well lasers with ALE grown active regions have been demonstrated with laser thresholds as low as 400 A/cm2.

Hydrostatic pressure experiments on Al x Ga 1−x AsGaAs quantum-well heterostructure (QWH) laser diodes are described. Data are presented giving ∼11.5meV/kbar for the bandgap vs. pressure coefficient at lower pressures, with a change to 8.5–9meV/kbar at higher pressures. We suggest that this behavior is caused by biaxial and shear stresses in the active region induced by doping or composition mismatch relative to the confining layers, or between the n and p confining layers themselves. A model consistent with the experimental data is presented.

Data are presented on MO-CVD AlxGa1−xAs-GaAs multiple quantum-well heterostructures (QWH's) consisting of a large GaAs well (500 A) coupled to one or two arrays of smaller wells (50 A) that act, when photopumped, as phonon sources and resonators. These data show that the induced phonon-sideband laser operation of the larger well is quenched when the phonon-generating array, but not the larger well, is destroyed by thermal annealing. An estimate of the electron-phonon coupling coefficient for indirect optical transitions is presented, indicating enhanced coupling with increased phonon density and establishing a basis for stimulated phonon emission in QWH's.

Atomic layer epitaxy is a relatively new variation of conventional MOCVD in which the growth of III–V semiconductors takes place by alternately exposing the substrate to reactant precursors containing the elements of the compound(Pessa et. al., 1980; Pessa et. al., 1983; Nishizawa et. al, 1985; Tischler and Bedair, 1986). Under the appropriate growth conditions, saturated surface reactions can be made to dominate the growth so that one monolayer of the compound is formed per exposure cycle. Although this process is often carried out in a vacuum environment where only surface reactions are important, the highest quality materials have been obtained when carried out in near atmospheric pressure conditions. Under these circumstances it is more difficult to predict the relative roles of surface and gas phase reactions in the overall growth process. We have been involved in a group of studies that are directed toward understanding the roles of these processes in the ALE growth of GaAs. This paper will describe these studies and our current understanding of the near atmospheric pressure ALE growth of GaAs.

Data are presented showing that a GaAs quantum well, sandwiched between two epitaxial AlxGa1-xAs(x ∼ 0.4) confining layers, loses its effectiveness as a collector of excess carriers and as a source of recombination radiation for well dimensions Lz < 100 A. It is shown that this behavior is expected because of the difficulty in scattering carriers to the bottom of the quantum well as Lz → lp, the path length for scattering (LO phonon).

Formulas are presented to calculate the heating of charge carriers and the generation of nonequilibrium phonons by intense photoexcitation of quantum-well heterostructures. It is shown that the deviations from thermal equilibrium are more pronounced for quasi-two-dimensional structures than for bulk material. Supporting experimental data are given on an MO-CVD Al x Ga 1- x As-GaAs heterostructure consisting of a large GaAs quantum well ( L z ∼ 200 A) coupled to a phonon-generating array of seven small GaAs quantum wells ( L z ∼ 50 A).

Data are presented on two different photopumped multiple quantum-well AlxGa1-xAs-GaAs heterostructures showing that for a large enough number (6) of coupled GaAs quantum wells (L z ∽ 50 A ) laser operation occurs two LO phonons (2× h ω LO ) below the lowest confined-particle transitions (E1 or E1'), and for a small number of wells (2) at multiples of ħωLO in the ∽ 240 meV range from just below the L band edge to E 1 - h ω LO . It is argued that the strength of the electron-phonon interaction is enhanced in two dimensions and that the LO phonon number density Nq is upset (increased) in these quantum well heterostructures, which are grown by metalorganic chemical vapor deposition (MO-CVD).

A theoretical model for the distribution of charged carriers at the interface of a doped Al x Ga 1− x As-undoped GaAs heterostructure is evaluated. Assuming a triangular potential well at the interface, we obtain numerically, curves describing the Fermi energy, depletion width in the alloy, and average separation of impurities and carriers, and the electron density at the interface as functions of alloy layer composition and doping level. The results obtained can be applied to multiple heterointerface quantum well heterostructures and superlattices as well. Carrie concentration profile data on a 51-layer modulation doped QWH grown by MOCVD are presented and are discussed using the results of the model.

The threshold current density, Jth, of a quantum-well laser diode is calculated taking into account the quasi-two-dimensional nature of the heterostructure. The calculated value of Jth(T) for a quantum-well laser diode is found, in agreement with experiment, to be less temperature sensitive than that of a conventional double heterojunction laser. The step-like densities of states and the perturbed (hot) carrier distribution of a quasi-two-dimensional structure are responsible for the weaker temperature dependence. Supporting data on quantum-well AlxGa1-xAsGaAs heterostructure laser diodes grown by MO-CVD are presented showing that in the conventional expression Jth(T) = Jth(0) exp (T/T0), T0 can be as high as ∼ 437°C.

A comparison is made of the suitability of MOCVD and MBE for application to the fabrication of heterojunction devices. The fundamental processes that control the growth, and the materials and device results achieved to data by each process are reviewed.

The rate of decomposition of trimethylgallium (TMGa) and arsine is measured by sampled gas infrared absorption spectroscopy. The decomposition of TMGa is observed to go to completion at temperatures above 475°C as measured by the concentration of CH4 that is formed in the decomposition process. An activation energy of 58–62 kcal/mol is measured for the loss of the first methyl group. The decomposition of arsine is characterized by strong surface interactions. The activation energy for the decomposition of arsine in a quartz vessel is 34 kcal/mol. The activation energy for decomposition of AsH3 in the presence of GaAs wafers was found to be 18 kcal/mol. The implications of these results for the growth of GaAs by metalorganic chemical vapor deposition (MOCVD) are discussed.

Laser operation ofp-n junctions in GaAs doped with amphoteric impurities is demonstrated. The previous limitation to laser operation of such junctions caused by the broad spatial spread of the injected carriers (electrons), and their consequent low density at a given current level, can be overcome with a heterojunction reflecting barrier juxtaposed within a diffusion length of the junction. The effect of the barrier is great enough to allow significant stimulated emission to occur in GaAs (p-type GaAs : Ge and GaAs : Si) not previously lased in junction structures, and at current levels 10 × lower than those that have been previously inadequate.

A method is described for extending the use of bevel cross sectioning of semiconductor layers from the usual limits of 500–1000 to 80–100 A. The modified bevel cross sectioning described is used to view the 11 layers (6 GaAs and 5 AlGaAs, each ∼ 120 A) and the 7 layers (4 GaAs and 3 AlGaAs, each ∼ 80 A) of two multiple quantum-well AlxGa1−xAsGaAs (laser) heterostructures grown by metalorganic chemical vapor deposition (MO-CVD).