Abrupt Heterointerfaces Research Articles

Reverse temperature dependence of Ge surface segregation during Si-molecular beam epitaxy

Ge surface segregation on Si(100) and Si(111) substrates during silicon molecular beam epitaxy was studied using x-ray photoelectron spectroscopy. During growth, the Ge atoms were only partly incorporated into the growing layer and the residuals segregated on the grown surface. The Ge segregation phenomena increased to a maxima at around 450 °C in the case of the Si(100) substrates and at around 650 °C in the case of the Si(111) substrates. Segregation decreased above these temperatures. These reverse temperature dependencies enable not only good crystallinity of the heteroepitaxial layer, but also abrupt heterointerfaces. The incorporation coefficients for Si(111) were much larger than those for Si(100), which is well explained by a model based on surface migration.

Operator ordering in effective-mass theory for heterostructures. II. Strained systems.

A new kinetic-energy operator -1/2${\mathrm{\ensuremath{\Elzxh}}}^{2}$${\mathit{m}}^{\mathrm{\ensuremath{\alpha}}}$${\mathit{a}}^{\mathrm{\ensuremath{\delta}}}$\ensuremath{\nabla}${\mathit{m}}^{\mathrm{\ensuremath{\beta}}}$${\mathit{a}}^{\mathrm{\ensuremath{-}}2\mathrm{\ensuremath{\delta}}}$\ensuremath{\nabla}${\mathit{m}}^{\mathrm{\ensuremath{\alpha}}}$${\mathit{a}}^{\mathrm{\ensuremath{\delta}}}$ suitable for effective-mass treatment of strained heterostructures is introduced. Here, m(r) is the local effective mass and a(r) is the local lattice constant. By comparison of exact results with effective-mass results for solvable test models, values of \ensuremath{\alpha}, \ensuremath{\beta}, and \ensuremath{\delta}, which ensure asymptotic agreement, are determined. Based on qualitative similarities between the test models considered and realistic systems, the boundary conditions [(1/a)\ensuremath{\varphi}=continuous], [(a/m)\ensuremath{\varphi}'=continuous] are proposed for abrupt heterointerfaces for conduction-band states in strained systems. When a single-band effective-mass equation is applicable to hole states, however, we propose that the continuity of \ensuremath{\varphi} and (1/m)\ensuremath{\varphi}' should be imposed instead.

Two‐dimensional simulation of quantum well lasers

AbstractA versatile two‐dimensional simulator for various types of semiconductor lasers for both steady state and transients has been developed. The simulator is capable of spectral analysis of quantum‐well semiconductor lasers, such as gain‐spectrum analysis, as well as analysis of the two‐dimensional current flow and optical intensity patterns. The simulator is based on the drift‐diffusion model with full Fermi‐Dirac statistics for the transport equations as well as for the Poisson equation. Simulation of the thermionic emission current is required at the abrupt heterointerfaces of the quantum well. For the spectral analysis of quantum‐well lasers, we have used the photon rate equation for each Fabry‐Perot mode. For the optical intensity pattern, we have solved the two‐dimensional Helmholtz eigenvalue equation using the subspace iteration method. The transient simulation is done by the full backward‐Euler method in conjunction with the full Newton approach for the entire semiconductor equations. To demonstrate the simulator, a model GaAs‐AIGaAs graded‐index‐separate‐confinement‐heterostructure buried‐quantum‐well laser is analyzed.

Reverse Dependence of Ge Surface Segregation on Growth Temperature in Si-Mbe

ABSTRACTGe surface segregation on Si(100) and Si(111) substrates during silicon molecular beam epitaxy is studied as a function of growth temperature. During growth, the Ge atoms are partly incorporated into the growing layer and the residuals segregate on the grown surface. The incorporation coefficient cannot be expressed in terms of a simple thermal activation process. That is, the Ge segregation phenomena increase to maxima at around 450°C for the Si(100) substrates and at around 650°C for the Si(111) substrates. Segregation decreases above these temperatures. These reverse temperature dependences enable not only good crystallinity of the heteroepitaxial layer, but also abrupt heterointerfaces. The incorporation coefficients for Si(111) are much larger than those for Si(100), which is explained by a model based on surface migration.

Atomically Abrupt and Smooth Heterointerfaces: An Optical Investigation

ABSTRACTLuminescence spectra from quantum wells are routinely interpreted in terms of atomically smooth and atomically abrupt interfaces. Here we show that this interpretation is inconsistent with photoluminescence, photoluminescence excitation, and quantitative microscopic (chemical lattice imaging) results. We argue that the discussion of interfacial roughness in terms of “an island size” is too naive. A full characterization of an interface requires the description of a “roughness spectrum”, specifying the amplitude of the interfacial corrugation vs corrugation wavelength over the relevant length scale.

Abrupt heterointerfaces in Al0.35Ga0.65As/Al0.05Ga0.95As/Al0.35Ga0.65As quantum well structure grown by liquid-phase epitaxy

The Al0.35Ga0.65As/Al0.05Ga0.95As/Al0.35Ga0.65As quantum well structure with a 70-Å-thick well has been successfully fabricated by liquid-phase epitaxy at 685 °C. The transmission electron microscope cross-section technique is applied to study the uniformity of the quantum well and the transition width at the heterojunction interface. An interesting phenomenon observed for the first time during liquid-phase epitaxy growth is that the interface of Al0.05Ga0.95As grown on Al0.35Ga0.65As is rather flat and sharp whereas the opposite growth sequence gives rise to a wavy interface. The transition width is less than 10 Å at the Al0.05Ga0.95As/Al0.35Ga0.65As interface but is about 30–50 Å at the inverted Al0.35Ga0.65As/Al0.05Ga0.95As interface. These values are much less than those reported in the literature. The peak energy of the photoluminescence spectrum reveals the quantum size effect.

Metalorganic chemical vapor deposition of InGaAsP/InP layers and fabrication of 1.3-μm planar buried heterostructure lasers

High-quality InGaAsP and InP layers were obtained with good surface morphology by low-pressure metalorgainic chemical vapor deposition (MOCVD). The growth apparatus could be relatively simple using trimethylindium (TMI) as indium source. Abrupt heterointerfaces were also obtained and the compositional grading at the interfaces was estimated to be less than 20–30 Å owing to the low-pressure growth technique. The selective burying growth on several types of mesas was investigated. The growth rate on the {111}B face was found to be smaller than that on the {111}A and the {100} face in MOCVD. As a result, very smooth and flat varying layers were obtained when the rectangular mesas were formed along the 〈110〉 direction. Using these characteristics, low-threshold 1.3-μm InGaAsP/InP lasers with planar buried heterostructures (PBH) were grown entirely by three-stage MOCVD. The threshold current Ith =10 mA and the differential quantum efficiency ηd =60%, which are comparable to lasers grown by liquid-phase epitaxy, were obtained. PBH lasers with multi-quantum-well active layers were also fabricated. Continuous operation with threshold current Ith =35 mA was achieved at room temperature.

A TEM study of GaInAsP/InP heterojunctions grown by atmospheric pressure MOVPE

GaInAsP/InP heterojunctions have been grown by atmospheric pressure MOVPE and examined by TEM. Defect free layers with thicknesses down to 10 Å have been observed, showing that very abrupt heterointerfaces are being achieved. Microscale roughness on quaternary layer surfaces has been observed for the first time in this materials system.

High-mobility two-dimensional electron gas in modulation-doped InAlAs/InGaAs heterostructures

The two-dimensional electron gas (2DEG) mobilities in MBE-grown modulation-doped InAlAs/InGaAs heterostructures have been studied. For the MBE growth, the flux intensity transients were minimized by adopting specially designed cell-shutters, which ensured highly abrupt heterointerfaces. A low temperature 2DEG mobility as high as 110000 cm 2/V s has been achieved with an optimized layer structure. The 2DEG mobilities are discussed in relation with relevant scattering mechanisms.

Reduced pressure MOVPE growth and characterization of GaAs/GaAlAs heterostructures using a triethylgallium source

The technologically important GaAs/GaAlAs materials system has been explored via reduced pressure MOVPE using a triethylgallium source. The growth system was designed for rapid responsein order to attain sharp interfaces and is load-locked to minimize contamination from atmospheric sources. GaAs/GaAlAs epilayers have been grown over the temperature range 625–750°C at pressure between 25 and 100 Torr. Photoluminescence (PL) measurements on GaAs and GaAlAs show sharp excitonic structure indicative of high purity. Hall and C - V carrier concentration profiling results confirm this. These high purity GaAs epilayers exhibit evidence of residual Mg acceptors and GaAlAs has a high radiative efficiency due to the use of an In-Ga-Al eutectic trap. Controlled doping experiments on multilayer structures have been performed using a SiH 4 dopant source and wide range dilution system. Epilayer surfaces are essentially featureless under interference contrast microscopic examination; however, “oval defects” originating at epilayer interfaces are still observed. Multiple quantum well (MQW) structures have been fabricated, without growth interruption, as a diagnostic aid in evaluating MOVPE system dynamic response. PL characterization of MQW structures with 6.5 nm well widths indicate high-quality abrupt heterointerfaces.

Improvement of the inverted GaAs/AlGaAs heterointerface

The luminescent properties of thin single quantum well AlxGa1−xAs/GaAs structures grown by molecular beam epitaxy were investigated. Since the photoluminescence response of these structures is dominated by the quality of the heterointerfaces, and particularly the inverted heterointerface, the effect of using several different types of interfaces was investigated. In structures with conventional abrupt heterointerfaces, only weak luminescence dominated by extrinsic processes was observed. By grading the Al mole fraction over ∼150 Å region at the heterointerface, an eightfold improvement in the excitonic photoluminescent intensity was obtained. By inserting a three-period superlattice of graded average composition, a 160-fold improvement in the excitonic photoluminescent intensity over the abrupt interface structure was obtained. The effect of using dimeric arsenic was also investigated. Since these structures provide feedback as to the quality of the heterointerfaces they can be used as a means of finding ways to improve them.

Improved AlxGa1−xAs bulk lasers with superlattice interfaces

A graded superlattice composed of AlxGa1−xAs and GaAs having a total thickness of 150 Å was incorporated on each side of the active layer of an otherwise standard double heterojunction laser. In spite of no optimization, current threshold densities as low as 600 A/cm2 at 880 nm were obtained in broad area devices with 300-μm cavity lengths. Otherwise identical structures with abrupt heterointerfaces in place of a graded superlattice exhibited current threshold densities in the range 1.2–1.4 kA/cm2. Silicon and beryllium were used for n- and p-type dopants. These results represent the best values for bulk laser and are attributed to improved interfaces primarily that between the bottom confining layer (substrate side) and the active layer.

Heterostructure devices: A device physicist looks at interfaces

The band offsets occurring at abrupt hetero-interfaces in heterostructure devices serve as potential steps acting on the mobile carriers, in addition to the macroscopic electrostatic forces already present in homostructure devices. Incorporation of hetero-interfaces therefore offers a powerful device design parameter to control the distribution and flow of mobile carriers, greatly improving existing kinds of devices and making new kinds of devices possible. Unusual device requirements can often be met by band lineups occurring in suitable semiconductor combinations. Excellent theoretical rules exist for the semi-quantitative ( < ± 0.2 eV) prediction of band offsets, even unusual ones, but no quantitatively accurate ( < ± 1 kT) purely theoretical predictive rules are currently available. Poorly-understood second-order nuisance effects, such as small interface charges and small technology-dependent offset variations, act as major limitations in device design. Suitable measurements on device-type structures can provide accurate values for interface physics parameters, but the most widely used measurements are of limited reliability, with pure I–V measurement being of least use. Many of the problems at interfaces between two III/V semiconductors are hugely magnified at interfaces between a compound semiconductor and an elemental one. Large interface charges, and a strong technology dependence of band offsets are to be expected, but can be reduced by deliberate use of certain unconventional crystallographic orientations. An understanding of such polar/nonpolar interfaces is emerging; it is expected to lead to a better understanding and control of III/V-only device interfaces as well.

MULTICHAMBER REACTORS : A SOLUTION TO THE PROBLEM OF GRADED HETEROINTERFACES IN HOT-WALL VPE SYSTEMS

The hydride growth process is well established as a technique for producing high quality homoepitaxial layers of the common III-V binary materials. However, it is difficult to obtain abrupt heterointerfaces for example between Ga In As and In P, as the gas speed is generally slow in order for the gas to follow the desired temperature profile in its passage through the source to substrate. Thus under normal conditions, a change of composition will occur in a time corresponding to the growth of several thousands of angstroms of material. This paper describes the growth of abrupt heterojunctions which have been achieved in a new geometry reactor, based on four separate growth chambers. In each chamber, the gas flow conditions for the growth of a specific composition are stabilised and changes of composition are achieved by rapidly moving the sample from one chamber to another. This new reactor has allowed us to grow multiple heterojunctions with an interface abruptness of better than 150 A.