Properties and use of ln0.5(AlxGa1-x)0.5P and AlxGa1-x as native oxides in heterostructure lasers

Data are presented demonstrating the formation of native oxides from high Al composition In_0.5(Al_xGa_1-x)_0.5P (x≳ 0.9) by simple annealing in a “wet” ambient. The oxidation occurs by reaction of the high Al composition crystal with H₂O vapor (in a N₂ carrier gas) at elevated temperatures (≥500° C) and results in stable transparent oxides. Secondary ion mass spectrometry (SIMS) as well as scanning and transmission electron microscopy (SEM and TEM) are employed to evaluate the oxide properties, composition, and oxide-semiconductor interface. The properties of native oxides of the In_0.5(Al_xGa_1-x)_0.5P system are compared to those of the Al_xGa_1-xAs system. Possible reaction mechanisms and oxidation kinetics are considered. The In_0.5(Al_xGa_1-x)_0.5P native oxide is shown to be of sufficient quality to be employed in the fabrication of stripe-geometry In_0.5(Al_xGa_1-x)_0.5P visible-spectrum laser diodes.     

Ohmic contacts ton-type In0.5Ga0.5P

The contact properties of alloyed Ni/Au-Ge/Mo/Au metallization to npoststagger+In_0.5Ga_0.5P epilayers grown by gas-source molecular beam epitaxy on GaAs substrates are reported. A minimum specific contact resistance of 10⁻⁵ Ωcm² was obtained forn = 2 × 10¹⁹ cm⁻³ material after alloying at 360° C for 20 sec. Above this temperature outdiffusion of lattice elements and reactions of the metallization with the In_0.5Ga_0.5P lead to severe morphological changes and degraded contact properties. From the temperature dependence of the contact resistance, thermionic emission was identified as the predominant current transport mechanism in these contacts.     

Ordering and disordering of doped Ga0.5In0.5P

The band gap of Ga_0.5In_0.5P is reported as a function of doping level and growth rate. The lowest band gaps are obtained for hole concentrations of about 2 × 10¹⁷ cm⁻³. For samples doped p-type above 1 × 10¹⁸ cm⁻³, the band gap increases dramatically, regardless of growth rate. This effect is shown to be the result of disordering during growth rather than a change in the equilibrium surface structure with doping. The doping level dependence of the band gap of Ga_0.5In_0.5P samples grown at higher and lower growth rates differs for selenium and zinc doping even though the effects of high doping are the same for both dopants.     

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.     

Relaxation of crystal lattice parameters and structural ordering in In x Ga1 − x As epitaxial alloys

Epitaxial In _x Ga_{1 − x} As/GaAs(100) heterostructures grown by the MOC-hydride method with a considerable lattice mismatch are studied by X-ray diffraction and scanning electron microscopy. The relaxation coefficient of the crystal lattice of the epitaxial alloy is calculated and the deformation energy is evaluated. It is shown that, at a concentration of the In atoms in metal sublattice close to x = 0.5, the superstructural phase formed on the surface of the epitaxial In _x Ga_{1 − x} As alloy is the InGaAs₂ compound with a layered tetragonal crystal lattice and ordered arrangement of the atoms of the metal sublattice in the growth plane of the epitaxial film.     

Diffuse diffracted features and ordered domain structures in GalnP layers grown by organometallic vapor phase epitaxy

Transmission electron diffraction (TED) and transmission electron microscope (TEM) studies have been made of organometallic vapor phase epitaxial Ga_xIn_1−xP layers (x ≈ 0.5) grown at temperatures in the range 570–690°C to investigate ordering and ordered domain structures. TED and TEM examination shows that the size and morphology of ordered domains depend on the growth temperature. The ordered domains change from a fine rod-like shape to a plate-like shape as the growth temperature increases. The domains are of width 0.6∼2 nm and of length 1∼10 nm. Characteristic diffuse features observed in TED patterns are found to depend on the growth temperature. Extensive computer simulations show a direct correlation between the ordered domain structures and such diffuse features. A possible model is suggested to describe the temperature dependence of the ordered domain structure.     

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.     

Phase formation under the effect of spinodal decomposition in epitaxial alloys of Ga x In1 − x P/GaAs(100) heterostructures

The effect of instability of alloys of Ga _x In_{1 − x} P/GaAs(100) semiconductor epitaxial heterostructures in the composition region x ≈ 0.50 is studied by X-ray diffraction and electron microscopy. The possibility of emergence of modulated relaxation structures on the surface of a Ga _x In_{1 − x} P alloy is shown. This phenomenon is accompanied by the emergence of satellites of main X-ray reflections corresponding to a single-phase structure.     

Towards high‐efficiency multi‐junction solar cells with biologically inspired nanosurfaces

Multi‐junction solar cells offer extremely high power conversion efficiency with minimal semiconductor material usage, and hence are promising for large‐scale electricity generation. However, suppressing optical reflection in the UV regime is particularly challenging due to the lack of adequate dielectric materials. In this work, bio‐inspired antireflective structures are demonstrated on a monolithically grown Ga_0.5In_0.5P/In_0.01Ga_0.99As/Ge triple‐junction solar cell, which overcome the limited optical response of reference devices. The fabricated device also exhibits omni‐directional enhancement of photocurrent and power conversion efficiency, offering a viable solution to concentrated illumination with large angles of incidence. A comprehensive design scheme is further developed to tailor the reflectance spectrum for maximum photocurrent output of tandem cells. Copyright © 2012 John Wiley & Sons, Ltd.     

The effect of temperature on the growth and properties of green light-emitting In0.5Ga0.5N films prepared by reactive sputtering with single cermet target

We have grown In_0.5Ga_0.5N films on SiO₂/Si (100) substrate at 100–400 °C for 90 min by rf reactive sputtering with single cermet target. The target was made by hot pressing the mixture of metallic indium, gallium and ceramic gallium nitride powder. X-ray diffraction (XRD) measurements indicated that In_0.5Ga_0.5N films had wurtzite structure and showed the preferential (1 0 -1 0) diffraction. Both SEM and AFM showed that In_0.5Ga_0.5N films were smooth and had small roughness of 0.6 nm. Optical properties were measured by photoluminescence (PL) spectra from room temperature to low temperature of 20 K. The 2.28 eV green emission was achieved at room temperature for all our InGaN films. The electrical properties of In_0.5Ga_0.5N films on a SiO₂/Si (100) substrate were measured by the Hall measurement at room temperature. InGaN films showed the electron concentration of 1.51×10²⁰–1.90×10²⁰ cm⁻³ and mobility of 5.94–10.5 cm² V⁻¹ s⁻¹. Alloying of InN and GaN was confirmed for the sputtered InGaN.     

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.     

Injection currents in mixed-layer Ga0.5In1.5S3 single crystals

Single crystals of Ga_0.5In_1.5S₃ are prepared by chemical transport reaction, and the currentvoltage characteristics and temperature dependence of the electrical conductivity are investigated. It is shown that the current transmission mechanism in an In-Ga_0.5In_1.5S₃-In structure is associated with monopolar injection. Fiz. Tekh. Poluprovodn. 31, 1011–1012 (August 1997)     

Plasma and wet chemical etching of In0.5Ga0.5P

Dry and wet chemical etching of epitaxial In_{0, 5}Ga_0.5P layers grown on GaAs substrates by gas-source molecular beam epitaxy have been investigated. For chlorine-based dry etch mixtures (PCl₃/Ar or CC1₂F₂/Ar) the etching rate of InGaP increases linearly with dc self-bias on the sample, whereas CH₄/H₂-based mixtures produce slower etch rates. Selectivities of ≥500 for etching GaAs over InGaP are obtained under low bias conditions with PCl₃/Ar, but the surface morphologies of InGaP are rough. Both CC1₂F₂/Ar and CH₄/H₂/Ar mixtures produce smooth surface morphologies and good (≥10) selectivities for etching GaAs over InGaP. The wet chemical etching rates of InGaP in H₃PO₄:HC1:H₂O mixtures has been systemically measured as a function of etch formulation and are most rapid (∼1 μn · min⁻¹) for high HCl compositions. The etch rate,R, in a 1:1:1 mixture is thermally activated of the formR ∝ , whereE _a = 11.25 kCal · mole⁻¹. This is consistent with the etching being reaction-limited at the surface. This etch mixture is selective for InGaP over GaAs.     

Drift velocity of electrons in quantum wells of selectively doped In0.5Ga0.5As/AlxIn1 ? xAs and In0.2Ga0.8As/AlxGa1 ? xAs heterostructures in high electric fields

The field dependence of drift velocity of electrons in quantum wells of selectively doped In_0.5Ga_0.5As/Al _x In_{1 − x} As and In_0.2Ga_0.8As/Al _x Ga_{1 − x} As heterostructures is calculated by the Monte Carlo method. The influence of varying the molar fraction of Al in the composition of the Al _x Ga_{1 − x} As and Al _x In_{1 − x} As barriers of the quantum well on the mobility and drift velocity of electrons in high electric fields is studied. It is shown that the electron mobility rises as the fraction x of Al in the barrier composition is decreased. The maximum mobility in the In_0.5Ga_0.5As/In_0.8Al_0.2As quantum wells exceeds the mobility in a bulk material by a factor of 3. An increase in fraction x of Al in the barrier leads to an increase in the threshold field E _th of intervalley transfer (the Gunn effect). The threshold field is E _th = 16 kV/cm in the In_0.5Ga_0.5As/Al_0.5In_0.5As heterostructures and E _th = 10 kV/cm in the In_0.2Ga_0.8As/Al_0.3Ga_0.7As heterostructures. In the heterostructures with the lowest electron mobility, E _th = 2–3 kV/cm, which is lower than E _th = 4 kV/cm in bulk InGaAs.     

High-temperature hysteretic electronic effects of (AlxGa1−x)0.5In0.5P (x>0.65)

We have studied (Al_xGa_1−x)_0.5In_0.5P doped with tellurium using deep level transient spectroscopy and associated electrical measurements. Several defect states are observed in the upper half of the band gap, that are believed to be intrinsic to the alloy system as well as related to the tellurium donors. Defects observed at measurement temperatures above 390 K exhibit a hysteretic behavior. The observed spectra depend on the biasing conditions applied to the Schottky diode during cooling. The hysteretic behavior suggests the existence of different defect configurations, which can be accessed under conditions of high temperatures and electrical stress, but remain stable below 300 K.     

Reciprocal space mapping of ordered domains in InxGa1−xP

We applied high resolution x-ray diffraction techniques to determine the three dimensional domain shape in nominally lattice-matched, CuPt_B ordered, In_xGa_1−xP epitaxial layers deposited on GaAs by metal organic vapor phase epitaxy. A technique of reciprocal space mapping is described which provides three-dimensional information on the shape and size of the ordered domains. The domain shape is obtained by reciprocal space mapping along orthogonal crystallographic directions. Applying this technique shows that, at large miscut angles, the domains are oriented close to the growth direction, and are elongated by differing extents along the [110] and [ ].     

Visible light-emitting diodes grown on optimized ∇x[InxGa1−x]P/GaP epitaxial transparent substrates with controlled dislocation density

Epitaxial transparent-substrate light-emitting diodes (ETS-LEDs) have been fabricated on optimized graded buffers of In_xGa_1−xP on GaP (∇_x[In_xGa_1−x]P/GaP) that feature controlled threading dislocation densities of 3×10⁶ cm⁻². The ETSLEDs show increasing efficiency from 575 nm to 655 nm, in marked contrast to previous reports where performance drops above 600 nm, and feature the lowest spectral widths ever reported in ∇_x[In_xGa_1−x]P/GaP. The improvement over earlier reports is attributed to large mean dislocation spacings in optimized ∇_x[In_xGa_1−x]P/GaP, which are an order of magnitude greater than the mean carrier diffusion length. A slight performance decline remains at 655 nm, but the overall performance of this first generation of ETS-LEDs is promising.     

Effect of thin strain-compensated Al0.6Ga0.4P layers on the growth of multiple-stacked InP/In0.5Al0.3Ga0.2P quantum dots

Multiple-stacked InP self-assembled quantum dots (SAQD or QD) were grown on an In_0.5Al_0.3Ga_0.2P matrix lattice-matched on a GaAs (001) substrate using metalorganic chemical vapor deposition. Cathodoluminescence (CL) scanning electron microscopy, and transmission electron microscopy were employed to characterize the optical, morphological, and structural properties of the grown QDs. We found that the CL line width broadens and the surface becomes rough with an increase in the number of stacked QD layers in the structure. However, by introducing thin tensile-strained Al_0.6Ga_0.4P layers in the middle of In_0.5Al_0.3Ga_0.2P spacer layers to compensate the compressive strain of the InP QD layers, the CL and morphology are significantly improved. Using this technique, 30-stacked InP/In_0.5Al_0.3Ga_0.2P QD structures with improved CL properties and surface morphology were realized.     

Quantitative and Depth-Resolved Investigation of Deep-Level Defects in InGaN/GaN Heterostructures

Deep-level defects in In_0.17Ga_0.83N/In_0.02Ga_0.98N/p-GaN:Mg heterostructures were studied using deep-level optical spectroscopy (DLOS). Depth-resolved DLOS was achieved by exploiting the polarization-induced electric fields to discriminate among defects located in the In_0.17Ga_0.83N and the In_0.02Ga_0.98N regions. Growth conditions for the In _x Ga_1−x N layers were nominally the same as those in InGaN/GaN multi-quantum-well (MQW) structures, so the defect states reported here are expected to be active in MQW regions. Thus, this work provides important insight into defects that are likely to influence MQW radiative efficiency. In_0.17Ga_0.83N-related bandgap states were observed at E _v + 1.60 eV and E _v + 2.59 eV, where E _v is the valence-band maximum, compared with levels at E _v + 1.85 eV, E _v + 2.51 eV, and E _v + 3.30 eV in the In_0.02Ga_0.98N region. A lighted capacitance–voltage technique was used to determine the areal density of deep states. The possible origins of the associated defects are considered along with their potential roles in light-emitting diodes.     

Photoluminescence studies on InGaAlP layers grown by low-pressure metalorganic chemical vapor deposition

The optical properties for In_0.5(Ga_1-x Al _x )_0.5P (0 <x < 0.4) layers, grown by low-pressure Metalorganic Chemical Vapor Deposition, have been studied with photolominescence (PL) measurement. The PL intensity decreases with the increase of the Al composition (0 <x < 0.4). This dependence could not be accounted for only by the electron overflow from theΓ band to the X band. And the PL intensity is directly proportional to the excitation power at low temperature, below 50 K. On the other hand, the PL intensity is proportional to the second power of the excitation power at a high temperature range (>200 K). These results indicate that non-radiative recombination centers bound to theΓ band in In_0.5(Ga_1-x Al _x )_0.5P play a very important role in the radiation mechanism. PL dependence also shows these non-radiative recombination centers are thought to have strong relation to the aluminum substitution for In_0.5(Ga_1-x Al _x )_0.5P.