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.     

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.     

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.     

Nonequilibrium room-temperature carrier distribution in InAs quantum dots overgrown with thin AlAs/InAlAs layers

The optical properties of quantum dots (QDs) formed in GaAs or Al_0.3Ga_0.7As matrices by overgrowth of initial InAs islands formed in the Stranski-Krastanov mode with thin AlAs/InAlAs layers have been studied. It is shown that no transport of carriers between the QDs occurs in the temperature range 10–300 K, so the carrier distribution is of a nonequilibrium nature. The thermal excitation of carriers from the QDs is suppressed by an increase in the energy spacing between the ground and excited states, absence of the level related to the wetting layer, and higher carrier localization energy in the QDs with respect to the continuum states when the Al_0.3Ga_0.7As matrix is used.     

Role of Si-doped Al<Subscript>0.3</Subscript>Ga<Subscript>0.7</Subscript>As layers in the high-frequency conductivity of GaAs/Al<Subscript>0.3</Subscript>Ga<Subscript>0.7</Subscript>As heterostructures under conditions of the quantum hall effect

The complex high-frequency conductivity of GaAs/Al_0.3Ga_0.7As heterostructures that are δ-doped and modulation-doped with silicon was investigated by acoustic methods under conditions of the integer quantum Hall effect. Both the real (σ₁) and imaginary (σ₂) parts of the complex conductivity σ(ω, H)=σ_i?iσ₂ were determined from the dependences of the absorption and velocity of surface acoustic waves on magnetic field. It is shown that, in the heterostructures with electron density n_s=(1.3–7)×10¹¹ cm^?2 and mobility μ=(1–2)×10⁵ cm²/(V s), the high-frequency conductivity near the centers of the Hall plateau is due to electron hopping between localized states. It is established that, with filling numbers 2 and 4, the conductivity of the Al_0.3Ga_0.7As:Si layer efficiently shunts the high-frequency hopping conductivity of the two-dimensional interface layer. A method of separating the contributions of the interface and Al_0.3Ga_0.7As:Si layers to the hopping conductivity σ(ω, H) is developed. The localization length of electrons in the interface layer is determined on the basis of the nearest neighbor hopping model. It is shown that, near the centers of the Hall plateau, both σ(ω, H) and n_s depend on the cooling rate of a GaAs/Al_0.3Ga_0.7As sample. As a result, the sample “remembers” the cooling conditions. Infrared light and static strain also change both σ(ω, H) and n_s. We attribute this behavior to the presence of two-electron defects (so-called DX^? centers) in the Al_0.3Ga_0.7As:Si layer.     

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.     

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.     

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.     

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.     

Structural and optical properties of InAs quantum dots in AlGaAs matrix

Structural and optical properties of InAs quantum dots (QDs) grown in a wide-bandgap Al_0.3Ga_0.7As matrix is studied. It is shown that a high temperature stability of optical properties can be achieved owing to deep localization of carriers in a matrix whose band gap is wider than that in GaAs. Specific features of QD formation were studied for different amounts of deposited InAs. A steady red shift of the QD emission peak as far as ∼1.18 μm with the effective thickness of InAs in Al_0.3Ga_0.7As increasing was observed at room temperature. This made it possible to achieve a much higher energy of exciton localization than for QDs in a GaAs matrix. To obtain the maximum localization energy, the QD sheet was overgrown with an InGaAs layer. The possibility of reaching the emission wavelength of ~1.3 μm is demonstrated. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 37, No. 5, 2003, pp. 578–582. Original Russian Text Copyright ? 2003 by Sizov, Samsonenko, Tsyrlin, Polyakov, Egorov, Tonkikh, Zhukov, Mikhrin, Vasil’ev, Musikhin, Tsatsul’nikov, Ustinov, Ledentsov.     

The formation of natural oxide on the mirrors of GaSb/GaInAsSb/GaAlAsSb laser heterostructures at places of emergence of al-rich layers

Formation of a natural oxide in ambient atmosphere of the mirrors of a GaSb/GaInAsSb/Ga_0.1Al_0.9As_0.93Sb_0.07 laser heterostructure at places of emergence Al-rich layers on the mirror surface is studied. The evolution of topography of the laser mirrors produced by cleavage under ambient and ultrahigh vacuum conditions was studied by AFM and STM for mirrors cleaved. The oxidation of Ga_0.1Al_0.9As_0.93Sb_0.07 layers on the mirror surface was monitored for more than a year. It is shown that the oxide layer develops over the course of several months and then attains a constant thickness of about 1 μm. In the course of natural oxidation, the volume of the oxide layer increases, which results in its protrusion above the laser mirror surface approximately by a third of the total thickness of the oxide formed. Ultrahigh vacuum studies ruled out the previously assumed occurrence of the effect of plastic extension of Al-rich layers at the instant of cleavage and the resulting protrusion over the cleavage plane. At the initial stage of oxidation, the oxide protruding above the surface demonstrates an unusual concave region in the upper part, which disappears as the oxide grows further. An explanation of the observed transformation of the shape of the oxide layer is offered. It is assumed that the chemical composition of the oxide varies as its thickness increases.     

p-Doping of GaAs and AlGaAs using the triethylamine adduct of dimethylzinc

The adduct between dimethylzinc and triethylamine has been used as a p-dopant source in the growth of GaAs and Al_0.3Ga_0.7As alloys by metalorganic vapour phase epitaxy (MOVPE). The dopling efficiency of this adduct in these alloys and in InP is lower than that of dimethyl zinc.     

Transient current spectroscopy and frequency dispersion studies of low temperature GaAs and Al0.3Ga0.7As metal-insulator-semiconductor diodes

Low temperature (LT)-grown GaAs and Al_0.3Ga_0.7As metal-insulator-n⁺-GaAs (MIN) diodes have been fabricated and their electrical properties analyzed. Studies were carried out to evaluate the interfacial quality of the LT layer and the underlying n⁺-GaAs layer using transient current spectroscopy (TCS) and capacitance-frequency (C-f) characterization. TCS studies on LT-GaAs revealed a high concentration of a continuum of states anda dominant electron trap with an activation energy of 0.52eV. In LT-Al_0.3Ga_0.7As, a shallow trap at 0.36eV and two deep level traps at 0.85eV and 1.12eV were observed. Frequency dispersion was observed to be less for LT-GaAs samples with an AlAs barrier layer than without an AlAs barrier layer. However, LT-Al_0.3Ga_0.7As MIN diodes displayed a smaller frequency dispersion than LT-GaAs MIN diodes. Upon further investigation into MISFET devices, it was found that LT-Al_0.3Ga_0.7As MISFET devices had better transconductance frequency dispersion characteristics than LT-GaAs MISFET devices did.     

Real-space electron transfer by thermionic emission in GaAsAlxGa1−xAs heterostructures: Analytical model for large layer widths

Calculations are presented for negative differential resistance (NDR) and switching in layered GaAsAl_xGa_1?xAs heterostructures with a high electric field parallel to the interface. The mechanism is based on thermionic emission of hot electrons from the GaAs layers into the Al_xGa_1?xAs layers. An analytical model is obtained in the limit of relatively large layer widths (400 Å or wider). The method of moments is employed to solve the Boltzmann equation, assuming a position-dependent electron temperature and Quasi-Fermi level in the Al_xGa_1?xAs layers, and a position-independent electron temperature and Quasi-Fermi level in the narrower GaAs layer. Thermal conduction of hot electrons from the GaAs layer into the Al_xGa_1?xAs layers is taken into account. The results of the calculations show that the threshold electric field for the onset of NDR and the peak-to-valley ratio can be controlled to a large extent by adjusting the mobility of the Al_xGa_1?xAs layer, the layer dimensions, and the potential barrier (Al mole fraction in the Al_xGa_1?xAs).     

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.     

Microstructural analysis of a Au/Pt/Pd/Zn ohmic contact to an AlGaAs/GaAs heterojunction bipolar transistor

Gold-based ohmic contacts, incorporating Pt, Pd, and Zn layers, to AIGaAs/GaAs heterojunction bipolar transistors (HBTs) have been characterized using transmission electron microscopy (TEM). The metallization was deposited onto a 30 nm graded emitter layer of n-type Al_xGa_1−xAs, which was on a 30 nm emitter layer of n-type Al_0.3Ga_0.7As, with the aim of contacting the underlying 80 nm thick graded base layer of p-type Al_xGa_1−xAs. Metal layers were deposited sequentially using electron beam evaporation and the resultant metallizations were annealed at temperatures ranging from 250-500°C for up to several minutes. A minimum contact resistance of ≈8.5 × 10⁻⁷ Ω-cm² was achieved, which corresponded to the decomposition of ternary phases at the metallization/semiconductor interface, to binary phases, i.e., PdGa and PtAs₂. Long term stability tests were done on the optimum contacts. Anneals at 270°C for up to four weeks in duration produced virtually no change in microstructure, with the exception of some outward diffusion of Ga and As.     

Energy spectrum of a nonideal quantum well in an electric field

The effect of an electric field on the energy spectrum of a quantum well with macroscopic fluctuations is studied. The Stark shift of the quasibound states in a quantum well and three field-dependent broadening mechanisms (field-induced homogeneous broadening and broadening due to well width and depth fluctuations) are calculated in a wide range of electric fields. As an example, the effect of an electric field on the energy spectrum of electrons in a 12-nm-wide GaAs/Al_0.3Ga_0.7As quantum well with 5% width and depth fluctuations is determined. Fiz. Tekh. Poluprovodn. 32, 1108–1113 (September 1998)     

Growth and evaluation of lpe graded composition AlxGa1−xAs layers for high efficiency graded bandgap solar cells

A melt-mixing LPE growth technique to obtain a graded composition Al_xGa_1−xAs layer is described. The graded bandgap AlGaAs/GaAs solar cell requires the Al fraction (x) in the Al_xGa_1−xAs surface layer to increase from the junction towards the surface. The graded composition Al_xGa_1−xAs layer creates a built-in electric field for minority carriers which improves the carrier collection process. This graded bandgap solar cell should enable one to realize the full potential of GaAs as a material for solar energy conversion. Quantitative evaluation of the composition profile for these graded layers is needed to develop the proper growth technique and to understand the resulting solar cell characteristics. Rutherford backscattering analysis technique is described which has been successfully used to profile such layers.     

Properties of Ge-doped GaAs and Alx Ga1−xAs,Sn-doped Alx Ga1−xAs and Si-Te-doped GaAs

In order to better understand the electrical and optical properties of GaAs and Al_xGa_1-x As used in making double heterojunction lasers, we have studied the Hall coefficient, resistivity and photoluminescence behavior of doped epitaxial samples of these materials. In particular, we report results on Ge-doped GaAs and Al_x Ga_1-x As, Sn-doped Al_xGa_1-x As and Si-Te-doped GaAs single crystal layers which were grown on GaAs substrates by the liquid phase epitaxial method. The effects of impurities in the solution on the carrier concentration, mobility, photoluminescence spectra and possible recombination processes in these layers are discussed.     

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.