The effect of built-in electric field in GaN/AlGaN quantum wells with high AIN mole fraction

Spontaneous and piezoelectric polarization in hexagonal GaN/AlGaN heterostructures give rise to large built-in electric fields. The effect of the builtin electric field in GaN/Al_xGa_1−xN quantum wells was investigated for x=0.2 to 0.8 by photoluminescence studies. The quantum well structures were grown by molecular beam epitaxy on (0001) sapphire substrates. Cross-sectional transmission electron microscopy performed on the samples revealed abrupt interfaces and uniform layer thicknesses. The low temperature (4 K) photoluminescence peaks were progressively red-shifted due to the quantum confined Stark effect depending on the AlN mole fraction in the barriers and the thickness of the GaN quantum well. Our results verify the existence of very large built-in electric fields of up to 5 MV/cm in GaN/Al_0.8Ga_0.2N quantum wells.     

Optical properties of hybrid quantum-well–dots nanostructures grown by MOCVD

The deposition of In _x Ga_1–x As with an indium content of 0.3–0.5 and an average thickness of 3–27 single layers on a GaAs wafer by metalorganic chemical vapor deposition (MOCVD) at low temperatures results in the appearance of thickness and composition modulations in the layers being formed. Such structures can be considered to be intermediate nanostructures between ideal quantum wells and quantum dots. Depending on the average thickness and composition of the layers, the wavelength of the photoluminescence peak for the hybrid InGaAs quantum well–dots nanostructures varies from 950 to 1100 nm. The optimal average In _x Ga_1–x As thicknesses and compositions at which the emission wavelength is the longest with a high quantum efficiency retained are determined.     

Investigation of the physical properties of semiconductor nanostructures using samples with laterally nonuniform layers: 1. Photoluminescence

A new technique for the experimental investigation of semiconductor structures is suggested and implemented. The technique is based on an analysis of correlations in the spectra of samples with laterally nonuniform layers. A molecular-beam-epitaxy-grown sample containing Al_xGa_1?xAs-GaAs and GaAs-In_yGa_1?yAs quantum wells (QWs) and a modulation doped Al_xGa_1?xAs-GaAs heterojunction was studied by photoluminescence (PL) spectroscopy at 77 K. The dependences of the PL spectra on the parameters describing sample nonuniformity were analyzed, which made it possible to characterize the processes of the charge-carrier redistribution in the structure and to reveal a number of specific features in the PL of narrow GaAs QWs. In the entire range of the nonuniformity-related variation in the semiconductor structure parameters, the values of the optical transition energies determined experimentally agree with those calculated theoretically and may serve as a basis for estimating these parameters. It is shown in this study that the suggested approach is highly informative, which stems from the capacity for precision control over the technologically adjustable parameters of the structure within the same sample.     

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.     

Molecular beam epitaxy of GaPN,GaPAsN, and InGaPN nitride solid solutions

Experimental samples of semiconductor heterostructures with GaP_1?x N _x layers and _{In1?x ? y} GaP _y N _x and GaP _y N _x As_{1 ? x ? y} quantum wells are synthesized by molecular beam epitaxy on GaP (001) substrates. The structural properties of the samples are investigated by X-ray diffraction and the molar fraction x of nitrogen in the GaP_{1 ? x} N _x layers is determined. To compare the structural and optical properties of the samples, the photoluminescence of epitaxial GaP_{1 ? x} N _x layers and heterostructures with InGaPN and GaPAsN quantum wells with GaPN barriers is studied. The photoluminescence and X-ray diffraction data on the GaP_1?x N _x samples are compared with parameters calculated within the band anticrossing model. Based on the experimental and calculated data, it is concluded that the hybridization parameter is not constant and depends on the molar fraction of nitrogen.     

Unambiguously Enhanced Ultraviolet Luminescence of AlGaN Wavy Quantum Well Structures Grown on Large Misoriented Sapphire Substrate

High‐quality epitaxy consisting of Al_1?xGa_xN/Al_1?yGayN multiple quantum wells (MQWs) with sharp interfaces and emitting at ≈280 nm is successfully grown on sapphire with a misorientation angle as large as 4°. Wavy MQWs are observed due to step bunching formed at the step edges. A thicker QW width accompanied by a greater accumulation of gallium near the macrostep edge than that on the flat‐terrace is observed on 4° misoriented sapphire, leading to the generation of potential minima with respect to their neighboring QWs. Consequently, a significantly enhanced photoluminescence intensity (at least ten times higher), improved internal quantum efficiency (six times higher at low excitation laser power), and a much longer carrier lifetime are achieved. Importantly, the wafer‐level output‐power of the ultraviolet light emitting diodes on 4° misoriented substrate is nearly increased by 2–3 times. This gain is attributed to the introduction of compositional inhomogeneities in AlGaN alloys induced by gallium accumulation at the step‐bunched region thus forming a lateral potential well for carrier localization. The experimental results are further confirmed by a numerical modeling in which a 3D carrier confinement mechanism is proposed. Herein, the compositional modulation in active region arising from the substrate misorientation provides a promising approach in the pursuit of high‐efficient ultraviolet emitters.     

Population inversion between Γ subbands in quantum wells under the conditions of Γ-<Emphasis Type="Italic">L</Emphasis> intervalley transfer

Monte Carlo simulations of electron transport in Al_xGa_1−x As/GaAs/In_yGa_1−y As double-quantum-well heterostructures in high lateral electric fields are carried out. It is shown that, under the conditions of intervalley Γ-L electron transfer, there exists a population inversion between the first and the second quantum-confinement subbands in the Γ valley. The population inversion appears in the fields exceeding 4 and 5.5 kV/cm at 77 and 300 K, respectively. The gain in a superlattice composed of such quantum wells is estimated to be on the order of 100 cm⁻¹ for radiation with a wavelength of 12.6 μm. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 37, No. 2, 2003, pp. 224–229. Original Russian Text Copyright ? 2003 by Aleshkin, Andronov, Dubinov.     

Quantum-confined stark effect and localization of charge carriers in Al0.3Ga0.7N/Al0.4Ga0.6N quantum wells with different morphologies

The electric fields in Al_0.3Ga_0.7N/Al_0.4Ga_0.6N quantum wells are estimated. The quantum wells are grown by plasma-assisted molecular-beam epitaxy with plasma activation of nitrogen. The three-dimensional and planar modes of buffer layer growth are used. The transition to the three-dimensional mode of growth yields a substantial increase in the photoluminescence intensity of the quantum wells and a shift of the photoluminescence line to shorter wavelengths. These effects are attributed to the fact that, because of the extra three-dimensional localization of charge carriers in the quantum-well layer, the quantum-confined Stark effect relaxes. The effect of localization is supposedly due to spontaneous composition fluctuations formed in the AlGaN alloy and enhanced by the three-dimensional growth.     

Low-temperature photoluminescence and X-ray diffractometry study of InxGa1−x As quantum wells

The structures grown by molecular-beam epitaxy with In_xGa_1?x As quantum wells (QWs) in GaAs were studied by X-ray diffractometry and low-temperature photoluminescence techniques. The inhomogeneity of the QW composition along the growth direction was established. Energy positions of the exciton recombination lines in the QWs with step-graded In distribution were calculated, and good agreement with the experimental data was obtained.     

1 550 nm long-wavelength vertical-cavity surface emitting lasers

A 1 550 nm long-wavelength vertical cavity surface emitting laser (VCSEL) on InP substrate is designed and fabricated. The transfer matrix is used to compute reflectivity spectrum of the designed epitaxial layers. The epitaxial layers mainly consist of 40 pairs of n-AlxGayIn(1-x-y)As/InP, and 6 strain compensated AlxGayIn(1-x-y)As/InP quantum wells on n-InP substrate, respectively. The top distributed Bragg reflection (DBR) mirror system has been formed by fabricating 4.5 pairs of SiO2/Si. The designed cavity mode is around 1 536 nm. The dip of the fabricated cavity mode is around 1 530 nm. The threshold current is 30 mA and the maximum output power is around 270 μW under CW operation at room temperature.     

Study of optical characteristics of structures with strongly strained In<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ga<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>As quantum wells

Results of photoluminescence (PL) studies of heterostructures with strongly strained InxGa_{1 ? x} As quantum wells (QWs) are presented. It is shown that the dependence of the PL intensity on the QW thickness has a maximum whose position depends on the composition of the In _x Ga_{1 ? x} As solid solution. The PL wavelength at the maximum intensity is 1.13 µm at a QW thickness of 60 µm at a QW thickness of 50 Å for x = 0.39 and 0.42, respectively.     

Characterization of AlxInyGa1−x−yN quaternary alloys grown on sapphire substrates by molecular-beam epitaxy

High-resolution X-ray diffraction (HR-XRD) with rocking curve, atomic force microscopy (AFM) and photoluminescence (PL) spectroscopy have been performed on high-quality quaternary Al_xIn_yGa_1−x−yN thin films at room temperature. The Al_xIn_yGa_1−x−yN films were grown on c-plane (0 0 0 1) sapphire substrates with AlN as buffer layers using a molecular beam epitaxy (MBE) technique with aluminum (Al) mole fractions x ranging from 0.0 to 0.2 and constant indium (In) mole fraction y=0.1. HR-XRD measurements confirmed the high crystalline quality of these alloys without any phase separation. The X-ray rocking curve of Al_xIn_yGa_1−x−yN films typically shows full widths at half maximum (FWHM) intensity between 14.4 and 28.8 arcmin. AFM measurements revealed a two-dimensional (2D) growth mode with a smooth surface morphology of quaternary epilayers. PL spectra exhibited both an enhancement of the integrated intensity and an increasing blueshift with increased Al content with reference to the ternary sample In_0.1Ga_0.90N. Both effects arise from Al-enhanced exciton localization. PL was used to determine the behavior of the energy band gap of the quaternary films, which was found to increase with increasing Al composition from 0.05 to 0.2. This trend is expected since the incorporation of Al increases the energy band gap of ternary In_0.1Ga_0.90N (3.004 eV). We have also investigated the bowing parameter for the variation of energy band gaps and found it to be very sensitive on the Al composition. A value of b=10.4 has been obtained for our quaternary Al_xIn_yGa_1−x−yN alloys.     

Evolution of Threading Dislocation Density and Stress in GaN Films Grown on (111) Si Substrates by Metalorganic Chemical Vapor Deposition

We have studied the evolution of threading dislocations (TDs), stress, and cracking of GaN films grown on (111) Si substrates using a variety of buffer layers including thin AlN, compositionally graded Al _x Ga_1-x N (0 ≤ x ≤ 1), and AlN/Al _y Ga_1-y N/Al _x Ga_1-x N (0 ≤ x ≤ 1, y = 0 and 0.25) multilayer buffers. We find a reduction in TD density in GaN films grown on graded Al _x Ga_1-x N buffer layers, in comparison with those grown directly on a thin AlN buffer layer. Threading dislocation bending and annihilation occurs in the region in the graded Al _x Ga_1-x N grown under a compressive stress, which leads to a decrease of TD density in the overgrown GaN films. In addition, growing a thin AlN/Al _y Ga_1-y N bilayer prior to growing the compositionally graded Al _x Ga_1-x N buffer layer significantly reduces the initial TD density in the Al _x Ga_1-x N buffer layer, which subsequently further reduces the TD density in the overgrown GaN film. In-situ stress measurements reveal a delayed compressive-to-tensile stress transition for GaN films grown on graded Al _x Ga_1-x N buffer layers or multilayer buffers, in comparison to the film grown on a thin AlN buffer layer, which subsequently reduces the crack densities in the films.     

Structure and optical properties of heterostructures based on MOCVD (Al x Ga1 − x As1 − y P y )1 − z Si z alloys

Epitaxial heterostructures produced by MOCVD on the basis of Al _x Ga_{1 ? x} As ternary alloys with the composition parameter x ≈ 0.20–0.50 and doped to a high Si and P atomic content are studied. Using the high-resolution X-ray diffraction technique, scanning electron microscopy, X-ray microanalysis, Raman spectroscopy, and photoluminescence spectroscopy, it is shown that the epitaxial films grown by MOCVD are formed of five-component (Al _x Ga_{1 ? x} As_{1 ? y} P _y )_{1 ? z} Si _z alloys.     

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.     

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.     

Ultra-thin GaxIn1−xAs/InP (0≤x≤0.47) layer growth by chemical beam epitaxy

Ga_xIn_1?xAs/InP (0≤x≤0.47) lattice-matched and compressively strained quantum wells were grown by all gas source chemical beam epitaxy (CBE). Their optical properties were investigated by photoluminescence (PL) and optical absorption measurements. The thinnest Ga_xIn_1?xAs layer was 6Å-thick (2 monolayers) for Ga_0.47In_0.53As and 3Å-thick (1 monolayer) for InAs. In PL measurements, we found that for strained materials (x<0.47) luminescence intensity dropped with decreasing barrier thickness. Optical absorption properties were measured at room temperature, and excitonic absorption peaks were clearly observed. The wavelengths of excitonic peaks were in good agreement with a theoretical estimation obtained by using an effective mass approximation including heavy and light hole energy splitting at the γ point.     

Effects of geometry,applied hydrostatic pressure and magnetic field on the electron–hole transition energy in a GaAs–Ga1−xAlxAs pillbox immersed in a system of Ga1−yAlyAs

In this work, we study the behavior of the electron–hole transition energy in a GaAs–Ga_1?xAl_xAs pillbox immersed in a system of Ga_1?yAl_yAs as a function of thickness of the ladder barrier potential for a fixed length of the pillbox, length of the pillbox, thickness of the ladder barriers and pillbox position in the host of Ga_1?yAl_yAs. The behavior of the electron–hole transition energy as a function of an applied hydrostatic pressure and an applied magnetic field is also studied. For both electron and hole we found that in the strong confinement regime (L?10 Å) energy of the ground state as function of the position of the pillbox relative to the ladder barrier potential presents a behavior similar to the binding energy of a hydrogenic impurity in quantum wells, quantum wires and quantum dots [L. Esaki, R. Tsu, IBM J. Res. Dev. 14 (1970) 61; G. Bastard, Phys. Rev. B 24 (1981) 4714; N. Porras-Montenegro, J. López-Gondar, L.E. Oliveira, Phys. Rev. B 43 (1991) 1824]. Electron–heavy hole transition energies increase with the applied magnetic field. Also, we have found that these transition energies, as a function of the applied hydrostatic pressure, present an excellent agreement with experimental reports by Venkateswaran et al. [phys. Rev. B 33 (1986) 8416].     

Wavelength tuning in strained layer InGaAs-GaAs-AlGaAs quantum well lasers by selective-area MOCVD

Selective-area growth and regrowth using conventional atmospheric pressure metalorganic chemical vapor deposition is investigated for wavelength tuning in strained layer In_xGa_1-xAsGaAs-Al_y Ga_1-yAs quantum well lasers. Growth inhibition from a silicon dioxide mask is the mechanism used for the selective-area growth rate enhancement. By varying the width of the oxide stripe opening, differences in the growth rate yield different quantum well thicknesses, and hence different lasing wavelengths for devices on the same wafer. Both two-and three-step growth processes are utilized for selective-area epitaxy of strained layer In_xGa_1-xAs-GaAs quantum well active regions, with lasers successfully fabricated from the three-step growth. Scanning electron microscopy and transmission electron microscopy indicate that the absence of an oxide mask during Al_yGa_1-yAs growth is essential for successful device operation. A wide wavelength tuning range of over 630Å is achieved for lasers grown on the same substrate.     

Cathodoluminescence and Raman scattering in Ga1−x AlxP epitaxial films

Low-temperature cathodoluminescence and Raman scattering of Ga_1?x Al_xP epitaxial layers (0≤x≤0.8) grown by liquid phase epitaxy on the GaP(100) substrate are studied. The obtained cathodoluminescence spectra indicate that the dependence of the indirect energy gap on the composition parameter x is nonlinear. This nonlinearity can be described by the parabolic function with the inflection parameter b=0.13. Raman scattering studies show that the phonon spectrum of Ga_1?x Al_xP consists of one (Al-P)-like vibrational mode and three (Ga-P)-like modes.