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.     

Application of photoluminescence spectroscopy to studies of In0.38Al0.62As/In0.38Ga0.62As/GaAs metamorphic nanoheterostructures

The results of studies of the surface morphology, electrical parameters, and photoluminescence properties of In_0.38Al_0.62As/In_0.38Ga_0.62As/In_0.38Al_0.62As metamorphic nanoheterostructures on GaAs substrates are reported. Some micron-sized defects oriented along the [011] and \([0\bar 11]\) directions and corresponding to regions of outcropping of stacking faults are detected on the surface of some heterostructures. The Hall mobility and optical properties of the samples correlate with the surface defect density. In the photoluminescence spectra, four emission bands corresponding to the recombination of charge carriers in the InGaAs quantum well (1–1.2 eV), the InAlAs metamorphic buffer (1.8–1.9 eV), the GaAs/AlGaAs superlattice at the buffer-substrate interface, and the GaAs substrate are detected. On the basis of experimentally recorded spectra and self-consistent calculations of the band diagram of the structures, the compositions of the alloy constituents of the heterostructures are established and the technological variations in the compositions in the series of samples are determined.     

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.     

Characteristics of Si3N4/GaAs metal-lnsulator-semiconductor interfaces with coherent Si/Al0.3Ga0.7As interlayers

Si₃N₄/GaAs metal-insulator-semiconductor (MIS) interfaces with Si(10Å)/ Al_0.3Ga_0.7As (20Å) interface control layers have been characterized using capacitance-voltage (C-V) and conductance methods. The structure was in situ grown by a combination of molecular beam epitaxy and chemical vapor deposition. A density of interface states in the 1.1 × 10¹¹ eV^-1 cm^-2 range near the GaAs midgap as determined by the conductance loss has been attained with an ex situ solid phase annealing of 600°C in N₂ ambient. A dip quasi-static C-V demonstrating the inversion of the minority-carrier verifies the decent interface quality of GaAs MIS interface. The hysteresis and frequency dispersion of the MIS capacitors were lower than 100 mV, some of them as low as 50 mV under a field swing of about ±2 MV/cm. The increase of the conductance loss at higher frequencies was observed when employing the surface potential toward conduction band edge, suggesting the dominance of faster traps. Self-aligned gate depletion mode GaAs metal-insulator-semiconductor field-effect transistors with Si/Al_0.3Ga_0.7As interlayers having 3 μm gate lengths exhibited a transconductance of about 114 mS/mm. The present article reports the first application of pseudomorphic Si/ Al_0.3Ga_0.7As interlayers to ideal GaAs MIS devices and demonstrates a favorable interface stability.     

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.     

Electrical characteristics of low temperature-Al0.3Ga0.7As

In this work, we present electrical characterizations of n⁺ GaAs/low temperature (LT)-Al_0.3Ga_0.7As/n⁺ GaAs resistor structures in which the LT layers are grown at nominal substrate temperatures of 250 and 300°C. The resistivity and V_tfl parameters of these LT-Al_0.3Ga_0.7As layers are compared with those of LT-GaAs and Al_0.3Ga_0.7As grown at a normal growth temperature of 720°C. Low-temperature Al_0.3Ga_0.7As layers exhibit resistivities as high as 10¹² ohm-cm, nearly four orders of magnitude higher than that of LT-GaAs, and V_tfl values as high as 45 V, over twice that of LT-GaAs. We also find that the LT-Al_0.3Ga_0.7As materials grown at 250 and 300°C appear to show opposite and contradictory trends with respect to resistivity and V_tfl. We propose that this result can be explained by residual hopping conduction in the 250°C material. Temperature dependent conductivity measurements confirm the presence of a hopping mechanism in LT-Al_0.3Ga_0.7As grown at 250°C and yield activation energies of 0.77 and 0.95 eV for LT-GaAs and LT-Al_0.3Ga_0.7As, respectively.     

A study of the electrical and optical properties of Si delta-doped GaAs layers grown by MBE on a (111)A GaAs surface misoriented toward the [2\bar 1\bar 1] direction

Electrical and photoluminescence studies of MBE-grown Si delta-doped GaAs structures at a varied partial pressure ratio P _As /P _Ga =γ on substrates with (111)Ga orientation and misoriented toward the [2 $\bar 1\bar 1$ ] direction have been performed. Hall effect measurements demonstrated that the conduction type changes from p to n on raising the As pressure (i.e., γ). The observed changes in the photoluminescence spectra are interpreted in terms of a kinetic approach based on different dangling bond densities on terraces and steps of the vicinal surface appearing on [2 $\bar 1\bar 1$ ] substrates misoriented toward the [2 $\bar 1\bar 1$ ] direction.     

Hydride Vapor-Phase Epitaxy of c-Plane AlGaN Layers on Patterned Sapphire Substrates

Growth of Al _x Ga_1?x N layers by hydride vapor-phase epitaxy on patterned sapphire substrates is investigated. The pattern consists of honeycombs which by their orientation and size promote the formation of coalesced c-plane-oriented Al _x Ga_1?x N layers with reduced crack density. The orientation of parasitic crystallites in the honeycomb openings is investigated using scanning electron microscopy and electron back-scatter diffraction. Crystallites with their [ $ \bar{1} $ $ \bar{1} $ .0] and [52.3] directions parallel to the vertical growth direction of the Al_0.3Ga_0.7N layer are observed and successfully overgrown by a 20-μm-thick fully coalesced c-plane-oriented layer.     

DX centers in Sn-doped Ga0.7Al0.3As

We have measured and analyzed the extended fine structure on the Sn K-shell x-ray absorption spectra of GaAs and Ga_0.7Al_0.3As doped with ∼5 x 10¹⁸ cm⁻³ Sn. Our results and their implications for the atomic structure of DX centers are discussed.     

MHEMT with a power-gain cut-off frequency of f max = 0.63 THz on the basis of a In0.42Al0.58As/In0.42Ga0.58As/In0.42Al0.58As/GaAs nanoheterostructure

The method of molecular-beam epitaxy is used to grow a In_0.42Al_0.58As/In_0.42Ga_0.58As/In_0.42Al_0.58As nanoheterostructure with a step-graded metamorphic buffer on a GaAs substrate. The root-mean-square value of the surface roughness is 3.1 nm. A MHEMT (metamorphic high-electron-mobility transistor) with a zigzag-like gate of a length of 46 nm is fabricated on the basis of this nanoheterostructure; for this MHEMT, the cutoff frequencies for the current and power gain are f _T = 0.13 THz and f _max = 0.63 THz, respectively.     

Diffusion of zinc into gaas through Al0.3Ga00.7As

The diffusion of zinc into GaAs, Al_0.3Ga_0.7As and Al_0.3Ga_0.7As/GaAs single heterostructures have been studied. The depth of the diffusion front is found to be proportional to the square root of the diffusion time, [t]^1/2, and for single heterostructures the Al_0.3Ga_0.7As layer thickness,d ₁ modifies this relationship through decreasing the junction depth byd ₁ multiplied by a constant. It is shown that this relationship can be used for predicting diffusion fronts in double heterostructures.     

Persistent photo-conductance and photoquenching of selectively doped Al0.3Ga0.7As GaAs/heterojunctions

The dependence on photon energy of the persistent photoconductivity (PPC) in selectively doped high mobility Al_0.3Ga_0.7As—GaAs heterostructures has been measured at temperatures below 80 K. A decrease in conductivity due to light exposure at one wavelength after exposure to light at another wavelength — photo-quenching — is also found. It is concluded that deep centers in GaAs and AlGaAs other than the DX center in AlGaAs are mainly responsible for PPC.     

Electrical behavior of modulation-and delta-doped Al x Ga1 − x As/In y Ga1 − y As/GaAs PHEMT structures

Modulation-and delta-doped Al_xGa_{1 ? x} As/In_yGa_{1 ? y} As/GaAs PHEMT structures are grown by MBE. The effect is examined of changes in the technique and level of doping on the electrical behavior of the structures. Photoluminescence spectroscopy combined with Hall-effect measurements is shown to be an effective strategy for the purpose. The experimental results are interpreted on the basis of calculated conductionband diagrams.     

Transferring of GaAs microtips using selective wet etching Al0.7Ga0.3As sacrificial layer

GaAs pyramidal microtips were successfully transferred from GaAs substrate to target wafer by a simple technique, i.e., selective wet etching off AlGaAs sacrificial layer. A GaAs/Al_0.7Ga_0.3As/GaAs sandwich structure is firstly formed on GaAs (0 0 1) substrate by metalorganic chemical vapor deposition, and then GaAs pyramidal microtips are grown on the sandwich structure using selective liquid-phase epitaxy. The GaAs microtips are removed from the sandwich structure by selective wet etching Al_0.7Ga_0.3As layer using concentrated HCl solution. Finally, the tips are glued onto the target wafer by a negative photoresist. During this transfer process the tips are completely encapsulated in a positive photoresist to protect against attack. Scanning electron microscopy images show that GaAs tips can be successfully transferred without any damage by this technique. The achievement reported here represents a significant step towards the application of scanning near-field optical microscopy.     

Dissociation of GaAs and Ga0.7Al0.3As during alloying of gold contact films

Although gold and gold-based alloys are frequently used in making ohmic contacts to Ga_xAl_1?xAs, the temperature- and time-dependence of the reactions which take place during the alloying procedure have not been extensively described previously. Using an evolved gas analysis (EGA)[11] with mass spectrometer, we have directly detected As which has passed through the thin gold contact film during alloying. With this technique, we have studied the dissociation of GaAs and Ga_0.7Al_0.3As during alloying as a function of processing temperature, time, and gold thickness.The total quantity of As released after prolonged annealing increases linearly with gold thickness, consistent with the idea that gold becomes “saturated” with Ga with respect to solid GaAs. An attempt to correlate the As evolved with the Au-Ga phase diagram is given. The amount of arsenic evolved and the rates of evolution were the same for gold on clean GaAs and GaAlAs.Changes in the surface preparation procedure prior to gold deposition had no effect on the As evolved from the Au/GaAs contact. On the Au/GaAlAs contact, poor or no predeposition cleaning lead to reduced As evolution.     

InAs-based heterostructure barrier varactor diodes with In0.3Al0.7As0.4Sb0.6 as the barrier material

InAs-based heterostructure barrier varactor (HBV) diodes with In_0.3Al_0.7As_0.4Sb_0.6 as the barrier material are demonstrated. Current–voltage and capacitance–voltage characteristics, as well as S-parameters, of HBV diodes with varying barrier thicknesses are examined. Maximum capacitance values and maximum-to-minimum capacitance ratios greater than those predicted by traditional HBV models were measured. The HBVs’ unconventional behavior in terms of charge accumulation layers adjacent to the wide bandgap barrier is discussed.     

Study of grown-in defects and effect of thermal annealing in Al0.3Ga0.7As and GaAs LPE layers

Studies of the grown-in deep-level defects in the undoped n-Al_xGa_1-xAs (x = 0.3) and GaAs epitaxial layers prepared by the liquid phase epitaxy (LPE) techniques have been made, using DLTS, I-V and C-V measurements. The effect of 300 °C thermal annealing on the grown-in defects was investigated as a function of annealing time. The results showed that significant reduction in these grown-in defects can be achieved via low temperature thermal annealing process. The main electron and hole traps observed in the Al_0.3Ga_0.7As LPE layer were due to the E_c-0.31 eV and E_v+0.18 eV level, respectively, while for the GaAs LPE layer, the electron traps were due to the E_c-0.42 and 0.60 eV levels, and the hole traps were due to E_v+0.40 and 0.71 eV levels. Research supported in part by the Air Force Wright Aeronautical Laboratories, Aeropropulsion Lab., Wright Patterson Air Force Base, Ohio, subcontract through SCEEE, contract F33615-81-C-2011, task-4, and in part by AFOSR grant no. 81-0187.     

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)     

Growth of Epitaxial (110) 0.7Pb(Mg<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>)O<Subscript>3</Subscript>-0.3PbTiO<Subscript>3</Subscript> Thin Films on r-Plane Sapphire Substrates by RF Magnetron Sputtering

The role of substrate temperature and substrate surface geometry in determining the crystal structure and crystallinity of 0.7Pb(Mg_1/3Nb_2/3)O₃-0.3PbTiO₃ (PMN-PT) thin films grown on r-plane sapphire substrates is examined. A 30-nm-thick amorphous PMN-PT seed layer deposited at 250°C and subjected to rapid thermal annealing at 850°C results in the formation of an epitaxial (110) perovskite PMN-PT growth template that can be used for subsequent growth of single-crystal (110) perovskite PMN-PT films at elevated temperature. The data show that single-crystal perovskite is promoted when the films nucleate with the \( \langle \overline{1} 11 \rangle \) PMN-PT direction parallel to the \( \langle 0\bar{2}21 \rangle \) Al₂O₃ direction.     

Hydrogen plasma passivation of bulk GaAs and Al0.3Ga0.7As/GaAs multiple-quantum-well structures on Si substrates

Hydrogen (H) plasma passivation effects on GaAs grown on Si substrates (GaAs on Si) are investigated in detail. H plasma exposure effectively passivates both the shallow and deep defects in GaAs on Si, which improves both the electrical and optical properties. It was found that the minority carrier lifetime is increased and the deep level concentration is decreased by the H plasma exposure. In addition, after H plasma exposure, room temperature photoluminescence (PL) for Al_0.3Ga_0.7As/GaAs multiple-quantum-well (MQW) on Si is enhanced with a decrease in the spectral width.