Improved area density and luminescence properties of InP quantum dots grown on In<Subscript>0.5</Subscript>Al<Subscript>0.5</Subscript>P by metal-organic chemical vapor deposition

We report on the growth of InP self-assembled quantum dots (QDs) on In_0.5Al_0.5P matrices by metal-organic chemical vapor deposition (MOCVD) on (001) GaAs substrates. The effects of the growth temperature and V/III-precursor flow ratio on the areal density and the cathodoluminescence (CL) properties of the grown QDs were systematically studied. We found that, when the growth temperature is ≤630°C, coherent QDs as well as large dislocated InP islands can be observed on the matrix surface. However, by using a two-step growth method, i.e., by growing the InAlP matrix layer at higher temperatures and growing InP QDs at lower temperatures, the formation of large dislocated islands can be effectively suppressed. Moreover, the areal density of the InP QDs is increased as the QD growth temperature is reduced. Furthermore, we found that the V/III ratio used in growing QDs and in growing the InAlP matrix layers has a quite different effect. In growing QDs, decreasing the V/III ratio results in an increase in the CL intensity and a decrease in CL line width; while in growing the InAlP matrix layers, increasing the V/III ratio results in an increase in the CL intensity of the InP QDs.     

Super-flat interfaces in In0.53Ga0.47As/In0.52Al0.48As quantum wells grown on (411)A InP substrates by molecular beam epitaxy

Effectively atomically flat interfaces over a macroscopic area (“(411)A super-flat interfaces”) were successfully achieved in In_0.53Ga_0.47As/In_0.52Al_0.48As quantum wells (QWs) grown on (411)A InP substrates by molecular beam epitaxy (MBE) at a substrate temperature of 570°C and V/III=6. Surface morphology of the In_0.53Ga_0.47As/In_0.52Al_0.48As QWs was smooth and featureless, while a rough surface of those simultaneously grown on a (100) InP substrate was observed. Photoluminescence (PL) linewidths at 4.2 K from the (411)A QWs with well width of 0.6–12 nm were 20–30 % narrower than those grown on a (100) InP substrate and also they are almost as narrow as each of split PL peaks for those of growth-interrupted QWs on a (100) InP substrate. In the case of the (411)A QWs, only one PL peak with very narrow linewidth was observed from each QW over a large distance (7 mm) on a wafer.     

Electrical characterization of self-assembled In0.5Ga0.5As/GaAs quantum dots by deep level transient spectroscopy

We have investigated electron emission from self-assembled In_0.5Ga_0.5As/GaAs quantum dots (QDs) grown by molecular-beam epitaxy (MBE). Through detailed deep level transient spectroscopy comparisons between the QD sample and a reference sample, we determine that trap D, with an activation energy of 100 meV and an apparent capture cross section of 5.4×10⁻¹⁸ cm², is associated with an electron quantum level in the In_0.5Ga_0.5As/GaAs QDs. The other deep levels observed, M1, M3, M4, and M6, are common to GaAs grown by MBE.     

Growth and properties of digitally-alloyed AlGaInP by solid source molecular beam epitaxy

Digital alloying using molecular beam epitaxy (MBE) was investigated to produce AlGaInP quaternary alloys for bandgap engineering useful in 600-nm band optoelectronic device applications. Alternating Ga_0.51In_0.49P/Al_0.51In_0.49P periodic layers ranging from 4.4 monolayers (ML) to 22.4 ML were used to generate 4,000-Å-thick (Al_0.5Ga_0.5)_0.51In_0.49P quaternary materials to understand material properties as a function of constituent superlattice layer thickness. High-resolution x-ray diffraction (XRD) analysis exhibited fine satellite peaks for all the samples confirming that digitally-alloyed (Al_0.5Ga_0.5)_0.51In_0.49P preserved high structural quality consistent with cross-sectional transmission electron microscopy (X-TEM) images. Low-temperature photoluminescence (PL) measurements showing a wide span of luminescence energies ～ 170 meV can be obtained from a set of identical composition digitally-alloyed (Al_0.5Ga_0.5)_0.51In_0.49P with different superlattice periods, indicating the bandgap tunability of this approach and its viability for III-P optoelectronic devices grown by MBE.     

Strain induced lateral ordering in Ga0.22In0.78As/Ga0.22In0.78P short period superlattices on (001) InP

The application of the strain induced lateral ordering process to the strain-compensated (Ga_0.22In_0.78As)_m (Ga_0.22In_0.78P)_m short period superlattices is investigated. The superlattices have been grown at low temperatures by solid source molecular beam epitaxy (MBE) on (001) InP. These superlattices have been used in multiquantum well heterostructures using InP as barriers. The anisotropic polarization of photoluminescence shows the existence of lateral modulation. Dark-field images using the 220 reflection gives modulated contrast in the superlattice layers. High-resolution transmission electron microscopy shows local variations of the interplanar spacing of the (200) planes as well as the angles they form with the (002) planes.     

Hall and photoluminescence studies of effects of the thickness of an additional In0.3Ga0.7As layer in the center of In0.15Ga0.85As/Al0.25Ga0.75As/GaAs high electron mobility transistors

In order to reduce the noise and carrier–donor scattering and thereby increase the carrier mobility of the pseudomorphic AlGaAs/InGaAs high electron mobility transistors (pHEMTs), we have grown Al_0.25Ga_0.75As/In_0.15Ga_0.85As/In_0.3Ga_0.7As/GaAs pHEMTs with varied In_0.3Ga_0.7As thickness, and studied the effects of the In_0.3Ga_0.7As thickness on the electron mobility and sheet density by Hall measurements and photoluminescence measurements. We calculated the electron and hole subbands and obtained good agreement between calculated and measured PL energies. It was found that the additional In_0.3Ga_0.7As layer could be used to reduce the carrier–donor scattering, but due to the increased interface roughness as the In_0.3Ga_0.7As layer becomes thicker, the interface scattering reduced the electron mobility. An optimal thickness of the In_0.3Ga_0.7As was found to be 2 nm.     

Thermal characteristics of InP, InAlAs, and AlGaAsSb metamorphic buffer layers used in In0.52Al0.48As/In0.53Ga0.47As heterojunction bipolar transistors grown on GaAs substrates

In_0.52Al_0.48As/In_0.53Ga_0.47As heterojunction bipolar transistors (HBTs) were grown metamorphically on GaAs substrates by molecular beam epitaxy. In these growths, InAlAs, AlGaAsSb, and InP metamorphic buffer layers were investigated. The InAlAs and AlGaAsSb buffer layers had linear compositional grading while the InP buffer layer used direct binary deposition. The transistors grown on these three layers showed similar characteristics. Bulk thermal conductivities of 10.5, 8.4, and 16.1 W/m K were measured for the InAlAs, AlGaAsSb, and InP buffer layers, as compared to the 69 W/m K bulk thermal conductivity of bulk InP. Calculations of the resulting HBT junction temperature strongly suggest that InP metamorphic buffer layers should be employed for metamorphic HBTs operating at high power densities.     

The effect of selenium doping on the optical and structural properties of Ga0.5ln0.5P

Selenium doping at an electron concentration of 10¹⁸ – 10¹⁹ cm⁻³ is shown to cause an increase in both the band gap and the disorder of Ga_0.5In_0.5P films grown by metalorganic chemical vapor deposition on GaAs substrates. The effect of selenium is shown to be very similar to that of the p-type dopants, zinc and magnesium. Selenium doping is also shown to have a dramatic smoothing effect on the surface morphology of Ga_0.5In_0.5P films.     

Effects of trimethylindium on the purity of In0.5Al0.5P and In0.5Al0.5as epilayers grown by metalorganic chemical vapor deposition

In_0.5Al_0.5P lattice-matched to GaAs and In_0.5A1_0.5As lattice-matched to InP epilayers were grown by atmospheric pressure metalorganic chemical vapor deposition (AP-MOCVD). The effect of trimethylindium on the purity of the as-grown layers was systematically studied using secondary ion mass spectroscopy (SIMS), deep level transient spectroscopy (DLTS), and capacitance-voltage (C-V) measurements. The SIMS results showed that oxygen is the main impurity in all layers and the oxygen concentration in InAlP was approximately one to four orders of magnitude higher than the oxygen concentration found in InALAs when the same indium source was used, indicating that more oxygen was introduced by the phosphine source than by the arsine source. Two electron traps in the InAlP epilayers and four electron traps in the InALAs epilayers were observed in this study. When a high-purity indium source was used, the best InAlP epilayer showed only one deep electron trap at 0.50 eV while the best InALAs epilayer showed no deep levels measured by DLTS. In addition, we also found that a high concentration of oxygen is related to the high resistivity in both material systems; this suggests that semi-insulating (SI) materials can be achieved by oxygen doping and high quality conducting materials can only be obtained through the reduction of oxygen. The oxygen concentration measured by SIMS in the best InALAs epilayer was as low as 3 × 10¹⁷ cm⁻³.     

Nanorelief of an oxidized cleaved surface of a grid of alternating Ga0.7Al0.3As and GaAs heterolayers

The morphology of an oxidized cleaved surface of a grid of alternating GaAs and Ga_0.7Al_0.3As layers was investigated by atomic-force microscopy. It was found that the surface of the native oxide film on a cleaved surface possesses a quasistationary nanorelief that reflects the composition of the layers of the heterostructure. The oxide regions above the GaAlAs layers are 0.5 nm higher than the regions above the GaAs layers. Etching off the oxide film shows that a nanorelief, which is inverted with respect to the relief of the oxide surface, also forms on the bared cleaved surface. The appearance of nanoreliefs on the surface and at the bottom boundary of the native oxide film is explained by the different oxidation depths of GaAs and Ga_0.7Al_0.3As and by an oxidation-induced increase in the volume. Fiz. Tekh. Poluprovodn. 33, 594–597 (May 1999)     

Effect of GaAs (100) substrate misorientation on the electrical parameters and surface morphology of metamorphic In0.7Al0.3As/In0.75Ga0.25As/In0.7Al0.3As HEMT nanoheterostructures

The results of studies of the effect of GaAs (100) substrate misorientation on the electrical parameters and surface morphology of high electron mobility In_0.7Al_0.3As/In_0.75Ga_0.25As/In_0.7Al_0.3As/GaAs nanoheterostructures are reported. Using molecular-beam epitaxy, two identical structures with a stepped compositional profile of the metamorphic In _x Al_{1 ? x} As (Δ _x = 0.05) buffer are grown on substrates of two types: a singular GaAs substrate with the orientation (100) ± 0.5° and a GaAs (100) substrate misoriented by (2 ± 0.5)° in the $\left[ {0\bar 1\bar 1} \right]$ direction. It is found that, in the case of the misoriented substrate, the concentration of the two-dimensional electron gas is ～40% higher. Broadening of the photoluminescence spectra and a shift of the peaks to lower photon energies, as experimentally observed in the case of the misoriented substrate, are attributed to the increased roughness of the heterointerfaces and strengthened fluctuations of the quantum-well width.     

Photoluminescence studies of In0.7Al0.3As/In0.75Ga0.25As/In0.7Al0.3As metamorphic heterostructures on GaAs substrates

The influence of the design of the metamorphic buffer of In_0.7Al_0.3As/In_0.75Ga_0.25As metamorphic nanoheterostructures for high-electron-mobility transistors (HEMTs) on their electrical parameters and photoluminescence properties is studied experimentally. The heterostructures are grown by molecular-beam epitaxy on GaAs (100) substrates with linear or step-graded In _x Al_{1 ? x} As metamorphic buffers. For the samples with a linear metamorphic buffer, strain-compensated superlattices or inverse steps are incorporated into the buffer. At photon energies ?ω in the range 0.6–0.8 eV, the photoluminescence spectra of all of the samples are identical and correspond to transitions from the first and second electron subbands to the heavy-hole band in the In_0.75Ga_0.25As/In_0.7Al_0.3As quantum well. It is found that the full width at half-maximum of the corresponding peak is proportional to the two-dimensional electron concentration and the luminescence intensity increases with increasing Hall mobility in the heterostructures. At photon energies ?ω in the range 0.8–1.3 eV corresponding to the recombination of charge carriers in the InAlAs barrier region, some features are observed in the photoluminescence spectra. These features are due to the difference between the indium profiles in the smoothing and lower barrier layers of the samples. In turn, the difference arises from the different designs of the metamorphic buffer.     

Dry etching characteristics of In1-x-y Ga x Al y as alloys in CCl2F2:Ar and CH4:H2:Ar discharges

The etching characteristics of InGaAlAs alloys lattice-matched to InP were investigated using low pressure (1 mTorr) electron cyclotron resonance CH₄:H₂:Ar or CCl₂F₂:Ar discharges with additional radiofrequency biasing of the samples. Using CCl₂F₂:Ar discharges with ≥250V negative bias it is possible to obtain equi-rate etching of the material for all compositions between In_0.53Ga_0.47As and In_0.52Al_0.48As. At lower bias values, formation of A1F₃ on the surface leads to an inhibition of the etch rates. By making use of the differential etch rates of InGaAlAs layers of different compositions in CH₄:H₂:Ar mixtures, it is possible to choose dc bias values that allow one to stop the etching at a pre-selected depth in a multi-layer structure. For example, for -150 V bias, one can etch through In_0.53Ga_0.47As, In_0.53Ga_0.40Al_0.07As and Ino.53Ga_0.30Al_0.17As layers, and stop at an underlying layer with composition In_0.53Ga_0.20Al_0.27As.     

MOCVD of Ga0.52In0.48P Using Tertiarybutylphosphine

Tertiarybutylphosphine was explored as a safer alternative to phosphine for the atmospheric pressure chemical vapor deposition of Ga_0.52In_0.48P on GaAs. For V/III ratios greater than or equal to 50 and growth temperatures between 630 and 700° C, the minority carrier diffusion length of Ga_0.52In_0.48P grown from t-butylphosphine, approached that of Ga_0.52In_0.48P grown from phosphine under similar conditions. Compared to deposition with phosphine, with t-butylphosphine the surface morphology was rougher, the carbon and sulfur contamination was greater, and the loss of indium by parasitic reaction was greater. The band gap of the Ga_0.52In_0.48P at fixed composition varied differently with V/III ratio.     

Origin of optical anisotropy in strained InxGa1−xAs/InP and InyAl1−yAs/InP heterostructures

Optical anisotropy in mismatched In_xGa_1−xAs/InP and In_yAl_1−yAs/InP heterostructures has been investigated by variable azimuthal angle ellipsometry and reflectance difference spectroscopy. The two approaches are shown to yield strongly correlated results. A comparison to high resolution x-ray diffraction, transmission electron microscopy, and atomic force microscopy studies indicates that large optical anisotropies are associated with surface roughening that results from three-dimensional growth. Our findings demonstrate that fast and nondestructive measurements of optical anisotropy can provide important information about the growth mode and crystalline quality of strained epitaxial layers. Formerly with Massachusetts Institute of Technology.     

Persistent photoconductivity and electron mobility in In0.52Al0.48As/In0.53Ga0.47As/In0.52Al0.48As/InP quantum-well structures

The influence of the width of the quantum well L and doping on the band structure, scattering, and electron mobility in nanoheterostructures with an isomorphic In_0.52Al_0.48As/In_0.53Ga_0.47As/In_0.52Al_0.48As quantum well grown on an InP substrate are investigated. The quantum and transport mobilities of electrons in the dimensionally quantized subbands are determined using Shubnikov-de Haas effect measurements. These mobilities are also calculated for the case of ionized-impurity scattering taking into account intersub-band electron transitions. It is shown that ionized-impurity scattering is the dominant mechanism of electron scattering. At temperatures T < 170 K, persistent photoconductivity is observed, which is explained by the spatial separation of photoexcited charge carriers.     

Thermal stability of Al0.48In0.52As/Ga0.47In0.53As/InP heterostructure and its improvement by phosphidization

A drastic decrease in the sheet carrier concentration of modulation-doped Al_0.48In_0.52As/Ga_0.47In_0.53As/InP heterostructures has been observed after O₂ plasma treatment followed by thermal treatment up to 350°C. The decrease in sheet carrier concentration, which is speculated to be caused by both plasma damage and impurities penetrating from the surface of the epilayer, can be suppressed substantially by using PH₃ plasma treatment prior to the O₂ plasma and thermal treatments.     

Effect of Si doping on the photoluminescence properties of AlGaInP/GaInP multiple quantum wells

The influence of Si doping on the photoluminescence (PL) properties of (Al_0.3Ga_0.7)_0.5In_0.5P/Ga_0.5In_0.5P multiple-quantum-wells (MQWs) was studied. For the samples without p-type layers, the PL peak wavelength from (Al_0.3Ga_0.7)_0.5In_0.5P/Ga_0.5In_0.5P MQWs did not vary when Si was doped in MQWs, the PL peak intensity did not change obviously and the PL FWHM broadened. We consider that Si doping results in worse interface quality of (Al_0.3Ga_0.7)_0.5In_0.5P/Ga_0.5In_0.5P MQWs. However, for the full light-emitting diode (LED) structure samples, the PL intensity of MQWs obviously increased when Si was doped in MQWs. The PL intensity from MQWs with Si-doped barriers was about 13 times stronger than that of undoped MQWs. The PL intensity from MQWs with Si-doped barriers and wells was strong as 28 times as that of undoped MQWs. The reasons are discussed.     

Crack-free Al0.5Ga0.5N epilayer grown on SiC substrate by in situ SiNx interlayer

The crystal quality and stress state of Al_0.5Ga_0.5N epitaxial layers on 6H-SiC wafers by introducing an in-situ deposited SiN_x nanomask layer grown by metal-organic chemical vapor deposition (MOCVD) were investigated. A SiN_x interlayer with various growth times was inserted to the Al_0.5Ga_0.5N epilayers. The full width at half maximum (FWHM) of X-ray diffraction peaks and the density of etch pits decreased dramatically by the SiN_x interlayer, indicating an improved crystalline quality. Also, it was found that the crack density and biaxial tensile stress in the Al_0.5Ga_0.5N film was significantly reduced by in situ SiN_x interlayer from optical microscopy, photoluminescence spectra and Raman spectra. Finally, a crack-free 1.8 μm thick Al_0.5Ga_0.5N epilayer grown on 6H-SiC substrate using the optimized SiN_x interlayer growth time was obtained.     

The effect of strained Al0.7In0.3As emitter layers on abrupt N-p+ AllnAs-GaInAs heterojunction diodes and heterojunction bipolar transistors

Strained Al_xIn_1−xAs/Ga_0.47In_0.53As heterojunction N-p⁺ diodes and heterojunction bipolar transistors (HBTs) have been grown on InP substrates by solid-source molecular-beam epitaxy, fabricated, and characterized. To determine the effects of the conduction-band discontinuity at the emitter-base heterojunction on turn-on voltage and ideality factor, a strained Al_0.7In_0.3As layer is inserted in the emitter near the base. Changes in transport across the junction are observed as a function of the strained-layer position and thickness. These results were used to implement strained emitter HBTs.