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.     

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.     

Study of new designs for the InAlAs metamorphic buffer on GaAs substrates with distributed compensation of elastic deformations

Two new designs for a metamorphic buffer, which are modifications of the In _x Al_{1 ? x} As metamorphic buffer due to groups of layers with differing lattice parameters, are proposed and implemented. This makes it possible to affect the relaxation of the metamorphic buffer. The structural and electrical characteristics of the obtained metamorphic HEMT nanoheterostructures are also studied.     

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.     

Ultrafast Dynamics of Photoexcited Charge Carriers in In<Subscript>0.53</Subscript>Ga<Subscript>0.47</Subscript>As/In<Subscript>0.52</Subscript>Al<Subscript>0.48</Subscript>As Superlattices under Femtosecond Laser Excitation

The results of experimental studies of the time dynamics of photoexcited charge carriers in In_0.53Ga_0.47As/In_0.52Al_0.48As superlattices grown by molecular-beam epitaxy on a GaAs substrate with a metamorphic buffer are reported. On the basis of the results of the numerical simulation of band diagrams, the optimal thickness of the In_0.52Al_0.48As barrier layer (4 nm) is chosen. At this thickness, the electron wave functions in In_0.53Ga_0.47As substantially overlap the In_0.52Al_0.48As barriers. This makes it possible to attain a short lifetime of photoexcited charge carriers (τ ~ 3.4 ps) at the wavelength λ = 800 nm and the pumping power 50 mW without doping of the In_0.53Ga_0.47As layer with beryllium. It is shown that an increase in the wavelength to λ = 930 nm (at the same pumping power) yields a decrease in the lifetime of photoexcited charge carriers to τ ~ 2 ps. This effect is attributed to an increase in the capture cross section of trapping states for electrons with lower energies and to a decrease in the occupancy of traps at lower excitation densities.     

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.     

Deep level investigation on n-In0.35Ga0.65As/GaAs structures

We have investigated the deep electronic levels in n-In_0.35Ga_0.65As epitaxial layers grown by molecular beam epitaxy (MBE) on graded In_xGa_1−xAs buffer/GaAs structures. In_xGa_1−xAs buffer layers with linear, parabolic and power composition grading law, respectively have been considered. The dependence of the deep levels distribution on the buffer grading law as well as on growth parameters such as the growth temperature and use of As₂ or As₄ beams is reported.     

Investigation of impact ionization and flicker noise properties in indium aluminum arsenide/indium gallinum arsenide metamorphic high electron mobility transistors with various work function-gate metals

This study investigates the effect of impact ionization using Ir, Pt, Pd, Ti gate metals and the direct correlation between these high work function metals and low frequency noise (LFN) on an In_0.4Al_0.6As/In_0.4Ga_0.6As metamorphic high electron mobility transistor (MHEMT). The effect of impact ionization on DC, RF, and cryogenic LFN is systematically studied and discussed. Gate metals with high work functions are used to suppress the kink effect and gate leakage current. Experimental results suggest that the Ir gate MHEMT exhibits superior thermal stable properties in a strong electrical field at various temperatures, associated with high gain, high current, and excellent low-frequency noise performance.     

In0.2Ga0.8As/GaAs quantum well laser with C doped cladding and ohmic contact layers

Carbon doping in Al_xGa_1−xAs was achieved using different approaches. The moderate growth temperature of 650°C was employed to grow C bulk-doped Al_xGa_1−xAs with a high Al mole fraction. The hole-density was altered using different V/III ratios. The trimethylaluminum (TMAl) was used as an effective C δ-doping precursor for growth of C δ-doped pipi doping superlattices in Al_xGa_1−xAs. the average hole-density of C δ-doped pipi superlattices was greater than 2−3 × 10¹⁹ cm⁻³. Zn-free GRINSCH In_0.2Ga_0.8As/GaAs laser structures were then grown using the C bulk-doped Al_xGa_1−xAs and C δ-doped pipi superlattice as a cladding and ohmic contact layer, respectively. The ridge waveguide laser diodes were fabricated and characterized to verify flexibility of these two doping approaches for device structures.     

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.     

Features of the diagnostics of metamorphic InAlAs/InGaAs/InAlAs nanoheterostructures by high-resolution X-ray diffraction in the ω-scanning mode

This study is devoted to the search for new possibilities of characterizing crystal-structure features using high-resolution X-ray diffraction. The emphasis is on the scanning mode across the diffraction vector (ω-scanning), since researchers usually pay little attention to this mode, and its capabilities have not yet been completely revealed. For the [011] and [01\(\bar 1\)] directions, the ω-peak half-width and the average tilt angle of the sample surface profile are compared. The diagnostic capabilities of X-ray scattering mapping are also studied. The objects of study are semiconductor nanoheterostructures with an InAlAs/InGaAs/InAlAs quantum well and an In_xAl_1–xAs metamorphic buffer grown by molecular-beam epitaxy on InP and GaAs substrates. Such nanoheterostructures are used to fabricate microwave transistors and monolithic integrated circuits. The objects under study are more completely characterized using the Hall effect, atomic-force microscopy, and low-temperature photoluminescence spectroscopy at 79 K.     

Effect of structural parameters on InGaAs/InAlAs 2DEG transport characteristics

The effect of structural parameters on the transport characteristics from 15 to 300 K of molecular beam epitaxy-grown InGaAs/InAlAs two dimensional electron gas structures lattice-matched to InP is determined. The InAlAs buffer layer thickness was varied from 1000 to 10,000Å. One sample also incorporated a InGaAs/InAlAs superlattice. The buffer layer thickness and structure had almost no effect on the mobility or sheet density. The InAlAs spacer layer was varied from 25 to 200Å. Increases in the InAlAs spacer thickness resulted in a monotonically decreasing sheet density and a peak in the mobility versus spacer thickness at 100Å. The highest 77 K mobility was 66,700 cm²/V/sds withN _D =1.2×10¹² cm^?2. The effect of illumination and temperature on the sheet concentration in these structures as well as on “bulk” InAlAs:Si was much smaller than in Al _x Ga_1?x As/GaAs structures or “bulk” Al _x Ga_1?x As, forx?0.30, indicating that devices based on this material system will not be characterized by many of the device instabilities observed in the AlGaAs/GaAs system.     

High density 2DEG in III-V semiconductor heterostructures and high-electron-mobility transistors based on them

Situation in high-electron-mobility transistor (HEMT) technology is discussed. The N-AlGaAs/InGaAs/GaAs pseudomorphic HEMT’s are now considered as most advanced for mmwave monolithic circuits, but metamorphic N-In_xAl_1−x As/In_yGa_1−y As/In_xAl_1−x As HEMT’s grown on GaAs substrates are very promising for the future high-frequency devices. High density 2DEG in HEMT’s is analyzed by means of the Hall effect and photoluminescence measurements. Processing technology of the sub-0.25-μm pseudomorphic HEMT’s, metamorphic HEMT’s and their characteristics are also described. Fiz. Tekh. Poluprovodn. 33, 1064–1065 (September 1999) This article was published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Interaction of electrons with optical phonons localized in a quantum well

The scattering rate of electrons in a quantum well by localized polar optical and interface phonons is considered. The dependence of the force of the electron-phonon interaction on the frequency of optical phonons in materials of the heterostructure forming the electron and phonon quantum wells is determined. It is shown that, by varying the composition of semiconductors forming the quantum well and its barriers, it is possible to vary the scattering rates of electrons by a factor of several times. The scattering rates of electrons by polar optical phonons are calculated depending on the fractions In _x and In _y in the composition of semiconductors forming the In _x Al_{1 ? x} As/In _y Ga_{1 ? y} As quantum wells. Dependences of the mobility and saturated drift velocity of electrons in high electric fields and quantum wells In _y Ga_{1 ? y} As on the composition of the In _x Al_{1 ? x} As barriers introduced into quantum wells are determined experimentally. The electron mobility increases, while the saturated drift velocity decreases as the fraction of In _x in the composition of barriers is increased.     

Determination of the thickness and spectral dependence of the refractive index of Al x In1 − x Sb epitaxial layers from reflectance spectra

A nondestructive method for measuring the thicknesses of epitaxial layers of Al _x In_{1 ? x} Sb alloys based on interference effects in reflectance spectra measured in a wide wavelength range (1–28 μm) is implemented. The studied 0.9–3.3 μm thick Al _x In_{1 ? x} Sb layers are grown on highly lattice-mismatched GaAs substrates by molecular beam epitaxy. The found thicknesses are in good agreement with the independent data of scanning electron microscopy. The spectral dependence of the refractive index n(E) of Al _x In_{1 ? x} Sb layers is measured both for the regions of transparency and fundamental absorption. The refractive index for the case of E < E ₀ was calculated by a double-oscillator model using a refined experimental dependence of the band gap on the composition E ₀(x). The experimental data on the n(E) of Al _x In_{1 ? x} Sb for energies E > E ₀ are found based on the interference pattern.     

S-Graded Buffer Layers for Lattice-Mismatched Heteroepitaxial Devices

We have conducted a theoretical study of the equilibrium strain and misfit dislocation density profiles for ??S-graded?? buffer layers of In _x Ga_1?x As on GaAs (001) substrates in which the compositional profile follows a normal cumulative distribution function. On the basis of this modeling work we show that the S-graded layer exhibits misfit dislocation-free regions near the substrate interface and the free surface (or device interface). The equilibrium peak misfit dislocation density as well as the thicknesses of the dislocation-free regions may be tailored by design of the compositional profile; this in turn should enable minimization of the density of electronically active threading dislocations at the top surface. S-graded buffer layers may therefore facilitate the achievement of metamorphic device structures with improved performance compared with similar structures having uniform or linearly graded buffers.     

Planarization and Processing of Metamorphic Buffer Layers Grown by Hydride Vapor-Phase Epitaxy

Hydride vapor-phase epitaxy (HVPE) is a high-growth-rate, cost-effective means to grow epitaxial semiconductor material. Thick HVPE-based metamorphic buffer layers (MBLs) can serve as “pseudosubstrates” with controllable lattice parameter. In our structures, the indium content in In _x Ga_1?x As is gradually increased from zero to the final composition corresponding to the desired lattice constant, and then a thick (～10 μm) constant-composition capping layer is grown. This thick capping layer promotes maximum strain relaxation while permitting use of polishing procedures to achieve surface planarity. Lattice-mismatched growth of MBLs invariably results in rough, cross-hatched surface morphology exhibiting up to 200 nm peak-to-valley roughness. This roughness can be eliminated by chemical mechanical planarization, thus creating a suitable surface for subsequent regrowth. Polishing of In _x Ga_1?x As is complicated by the sensitivity of the surface layer to the polishing parameters, particularly the applied pressure. Polishing at high applied pressure (12 psi) results in the formation of circular asperities hundreds of nanometers high and tens of microns in diameter. When lower applied pressure (4 psi) was used, the cross-hatching height of MBLs was lowered from 200 nm to <10 nm over a 350 μm lateral scale. The successfully planarized In_0.20Ga_0.80As MBLs were used as a substrate for a superlattice (SL) structure such as that used in quantum cascade lasers. Use of planarization before regrowth of the SL resulted in a reduction of the high-resolution x-ray diffraction peak full-width at half-maximum from 389″ to 159″.     

Resonant Raman scattering and atomic force microscopy of InGaAs/GaAs multilayer nanostructures with quantum dots

The transition from two-dimensional (2D) pseudomorphic growth to the three-dimensional (3D) (nanoisland) growth in In_xGa_1?xAs/GaAs multilayer structures grown by molecular-beam epitaxy was investigated by atomic force microscopy, photoluminescence, and Raman scattering. The nominal In content x in In_xGa_1?xAs was varied from 0.20 to 0.50. The thicknesses of the deposited In_xGa_1?xAs and GaAs layers were 14 and 70 monolayers, respectively. It is shown that, at these thicknesses, the 2D-3D transition occurs at x ≥ 0.27. It is ascertained that the formation of quantum dots (nanoislands) does not follow the classical Stranski-Krastanov mechanism but is significantly modified by the processes of vertical segregation of In atoms and interdiffusion of Ga atoms. As a result, the In_xGa_1?xAs layer can be modeled by a 2D layer with a low In content (x < 0.20), which undergoes a transition into a thin layer containing nanoislands enriched with In (x > 0.60). For multilayer In_xGa_1?xAs structures, lateral alignment of quantum dots into chains oriented along the \([\overline 1 10]\) direction can be implemented and the homogeneity of the sizes of quantum dots can be improved.     

Tuning the energy spectrum of InAs/GaAs quantum dots by varying the thickness and composition of the thin double GaAs/InGaAs cladding layer

It is shown that the ground state transition energy in quantum dots in heterostructures grown by atmospheric-pressure MOCVD can be tuned in the range covering both transparence windows of the optical fiber at wavelengths of 1.3 and 1.55 μm by varying the thickness and composition of the thin GaAs/In_xGa_1−x As double cladding layer. These structures also exhibit a red shift of the ground state transition energy of the In_xGa_1−x As quantum well (QW) as a result of the formation of a hybrid QW In_xGa_1−x As/InAs (wetting layer) between the quantum dots (QDs). The Schottky diodes based on these structures are characterized by an increased reverse current, which is attributed to thermally activated tunneling of electrons from the metal contact to QD levels. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 38, No. 4, 2004, pp. 448–454. Original Russian Text Copyright ? 2004 by Karpovich, Zvonkov, Levichev, Baidus, Tikhov, Filatov, Gorshkov, Ermakov.     

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.