Crack-free Al0.5Ga0.5N epilayer grown on SiC substrate by in situ SiNx interlayer

The crystal quality and stress state of Al_0.5Ga_0.5N epitaxial layers on 6H-SiC wafers by introducing an in-situ deposited SiN_x nanomask layer grown by metal-organic chemical vapor deposition (MOCVD) were investigated. A SiN_x interlayer with various growth times was inserted to the Al_0.5Ga_0.5N epilayers. The full width at half maximum (FWHM) of X-ray diffraction peaks and the density of etch pits decreased dramatically by the SiN_x interlayer, indicating an improved crystalline quality. Also, it was found that the crack density and biaxial tensile stress in the Al_0.5Ga_0.5N film was significantly reduced by in situ SiN_x interlayer from optical microscopy, photoluminescence spectra and Raman spectra. Finally, a crack-free 1.8 μm thick Al_0.5Ga_0.5N epilayer grown on 6H-SiC substrate using the optimized SiN_x interlayer growth time was obtained.     

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.     

Enhanced radiative recombination in AlGaN quantum wells grown by molecular-beam epitaxy

Dependences of the intensity of cathodoluminescence in multiple Al_0.55Ga_0.45N/Al_0.45Ga_0.55N quantum wells grown by molecular-beam epitaxy on the growth conditions are studied. An increase (by almost two orders of magnitude) in the intensity of the cathodoluminescence peak with an energy of 4.45 eV is observed as the quantum-well layer grows in the conditions of deep depletion with respect to ammonia. In this case, a tendency towards the mode of three-dimensional growth can be inferred from the pattern of diffraction of high-energy electrons; this effect is interpreted using a model of formation of AlGaN quantum dots.     

Characterization of MOVPE-grown (Al, In, Ga) N heterostructures by quantitative analytical electron microscopy

The low pressure metalorganic vapor phase epitaxy growth of wurzite (Al, In, Ga)N heterostructures on sapphire substrates is investigated by quantitative analytical scanning transmission electron microscopy techniques like atomic number (Z-) contrast imaging and convergent beam electron diffraction (CBED). Especially (In, Ga)N quantum wells of different thicknesses as well as superlattices were analyzed with respect to defects, chemical composition variations, interface abruptness and strain (relaxation) effects. The interfaces in In_0.12Ga_0.88N/GaN quantum wells appear to be asymmetric. Additionally, we found composition variations of Δx_In≥0.03 within the InGaN quantum wells. The application of electron diffraction techniques (CBED) yields quantitative information on strain and relaxation effects. For the case of 17 nm thick InGaN quantum wells, we observed relaxation effects which are not present in the investigated thin quantum wells of 2 nm thickness. The experimentally obtained diffraction patterns were compared to simulations in order to get values for strain within the quantum wells. Additionally, the influence of dislocations on the digression of superlattices is investigated.     

AlGaN-based quantum-well heterostructures for deep ultraviolet light-emitting diodes grown by submonolayer discrete plasma-assisted molecular-beam epitaxy

Features of plasma-assisted molecular-beam epitaxy of AlGaN compounds at relatively low temperatures of the substrate (no higher than 740°C) and various stoichiometric conditions for growth of the nitrogen- and metal-enriched layers are studied. Discrete submonolayer epitaxy for formation of quantum wells and n-type blocking layers without varying the fluxes of components was used for the first time in the case of molecular- beam epitaxy with plasma activation of nitrogen for the nanostructures with the Al _x Ga_{1 ? x} N/Al _y Ga_{1 ? y} N quantum wells. Structural and optical properties of the Al _x Ga_{1 ? x} N layers in the entire range of compositions (x = 0–1) and nanostructures based on these layers are studied; these studies indicate that there is photoluminescence at room temperature with minimum wavelength of 230 nm. Based on the analysis of the photoluminescence spectra for bulk layers and nanoheterostructures and their temperature dependences, it is concluded that there are localized states in quantum wells. Using the metal-enriched layers grown on the c-Al₂O₃ substrates, heterostructures for light-emitting diodes with Al _x Ga_{1 ? x} N/Al _y Ga_{1 ? y} N quantum wells (x = 0.4–0.5, y = x + 0.15) were obtained and demonstrated electroluminescence in the ultraviolet region of the spectrum at the wavelength of 320 nm.     

Metal-Semiconductor Nanoheterostructures with an AlGaN Quantum Well and In Situ Formed Surface Al Nanoislands

We report on fabrication and studies of composite heterostuctures consisting of an Al_0.55Ga_0.45N/A_l0.8Ga_0.2N quantum well and surface Al nanoislands, grown by plasma-assisted molecularbeam epitaxy on c-sapphire substrates. The influence of a substrate temperature varied between 320 and 700ºC on the size and density of the deposited Al nanoislands is evaluated. The effect of Al nanoislands on decay kinetics of the quantum well middle-ultraviolet photoluminescence has been investigated by time resolved photoluminescence. The samples with the maximum density of Al nanoislands of 10⁸ cm^–2 and lateral dimensions in the range of 100–500 nm demonstrated shortening of the photoluminescence lifetime, induced by interaction of the emitting quantum well and the plasmonic metal particles.

     

Polarization-resolved photoluminescence piezospectroscopy of GaAs/Al0.35Ga0.65As:Be quantum wells

The photoluminescence (PL) of GaAs/Al_0.35Ga_0.65As:Be quantum wells is studied at temperatures of 77 and 300 K under conditions of uniaxial compression along the [110] direction. There are two main lines in the PL spectra; at zero pressure and T = 77 K, the peaks appear at 1.517 and 1.532 eV. Comparison of the pressure dependences of the peak positions and the polarization of the PL measured experimentally with those calculated theoretically gives evidence that, at T ≥ 77 K, these bands originate from the recombination of free electrons with heavy and light holes in the GaAs valence band.     

Electrical and growth characteristics of Au/Al0.15Ga0.85N:Si structures with various Si incorporations

We have investigated the growth characteristics of n-Al_0.15Ga_0.85N:Si/GaN and the electronic properties of Au/n-Al_0.15Ga_0.85N:Si diode structures grown by metal-organic chemical vapor deposition (MOCVD) with various Si incorporations. The Al_0.15Ga_0.85N:Si layers were grown on undoped GaN/sapphire (0001) epitaxial layers in a horizontal MOCVD reactor at the reduced pressure of 300 torr. The mirrorlike surface, free of defects, such as cracks or hillocks, can be seen in the undoped Al_0.15Ga_0.85N epilayer, which was grown without any intentional flow of SiH₄. However, many cracks are observed in the n-Al_0.15Ga_0.85N:Si, which was grown with Si incorporation above 1.0 nmol/min. While Au/n-Al_0.15Ga_0.85N:Si diodes having low incorporation of Si showed retively good rectifying behavior, the samples having high Si incorporation exhibited leaky current-voltage (I-V) behavior. Particularly, the Au/n-Al_0.15Ga_0.85N:Si structure grown with Si incorporation above 1.0 nmol/min cannot be used for electrical rectification. Both added tunneling components and thermionic emission influence the current transport at the Au/n-Al_0.15Ga_0.85N:Si barrier when Si incorporation becomes higher.     

Al x Ga1 – x As/GaAs/Al x Ga1 – x As Double Quantum Well with a Thin AlAs Interwell Barrier: Structural Characterization by SIMS and XRD

An Al _x Ga_{1 – x} As/GaAs/Al _x Ga_{1 – x} As double quantum well with a thin AlAs interwell barrier is examined by SIMS and double-crystal XRD for an AlAs thickness of about 10 or 18 Å. Thickness and other structural parameters are determined for each layer. The rocking curves are found to indicate a fairly abrupt interwell barrier.     

Optical properties of strained layer (111)B Al0.15Ga0.85As-In0.04Ga0.96as quantum well heterostructures

Data presented here demonstrate that strained-layer (111)B Al_0.15Ga_0.85As-In_0.04Ga_0.96As quantum wells exhibit unique optical properties when compared to otherwise identical (100) oriented strained layers. Photoluminescence measurements identify a strain-induced electric field of order 6.7 Vμm^-1 within the (111)B quantum well that is not present for the (100) case. Photoluminescence excitation spectroscopy measurements show that the heavy-hole to light-hole energy band splitting is approximately 7 meV larger for the (111)B structure than for the (100) structure. Howard Hughes Doctoral Fellow IBM Graduate Fellow     

In x Ga 1–x As Strained-Layer Quantum Well in a Pseudomorphic Heterostructure: High-Resolution XRD Characterization for Different Quantum-Well Thicknesses

Strained-layer quantum wells of different thicknesses are realized in the form of an n-Al_0.25Ga_0.75As/In _x Ga_{1 – x} As/GaAs pseudomorphic heterostructure. The structural properties of the quantum wells are determined by high-resolution double-crystal XRD and by photoluminescence spectroscopy. The depth profile of In mole fraction is derived from the rocking curves. It indicates that the quantum-well interfaces are strongly smeared and hence their In mole fractions are less than the target value, 0.19. The XRD data are compared with the photoluminescence ones.     

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.     

Type I and Type II Alignment of the Light Hole Band in In0.15Ga0.85As/GaAs and in ln0.15Ga0.85As/Al0.15Ga0.85As Strained Quantum Wells

We present results of photoluminescence and cathodoluminescence measurements of strained undoped In_0.15Ga_0.85As/GaAs and In_0.15Ga_0.85As/Al_0.15Ga_0.85As quantum well structures, designed to throw light on the current controversy over light-hole band alignment at low In content. We compare these data with theoretical calculations of the confined state energies within the eight band effective mass approximation. Our analysis shows that for In_0.15Ga_0.85As/GaAs, the observed two transitions are consistent with either type I or type II alignment of the light hole band for band offset ratios within the accepted range. In the case of In_0.15Ga_0.85As/Al_0.15Ga_0.85As, however, our results clearly indicate type II alignment for the light hole band. We derive the band offset ratio Q, defined here as Q = δE_c/δE_g where δE_c is the conduction band offset and δE_g is the bandgap difference between the quantum well and the barrier in the presence of strain, for the In_0.15Ga_0.85As/Al_0.15Ga_0.85As system to be Q = 0.83 and discuss it in the context of the common anion rule.     

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.     

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.     

Effects of growth interruption on the optical properties of AIGaAs/GaAs and GaAs/InGaAs quantum wells grown by atmospheric pressure organometallic chemical vapor deposition

In this study, the effects of growth interruptions on Al_0.17Ga_0.83As/GaAs and GaAs/ In_xGa_1-xAs quantum wells (QWs) grown by organometallic chemical vapor deposition (OMCVD) were assessed using low-temperature photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopies. Growth interruption times were varied between 60, 10, and 0 sec. For both material systems, as the interruption time was reduced, the ground-state QW transition energies increased, while the linewidths of the peaks decreased. For the Al_0.17Ga_0.83As/GaAs structures, 5 K PL data suggests that the incorporation of impurities is enhanced by longer growth interruption times. In addition, as the interruption time was reduced, the energy separation between the 5 K PL and PLE peaks (Stokes shift) decreased, and was as low as 2.6 meV for no interruption. For GaAs/In_0.11Ga_0.89As samples, 2 K PL data indicated that the incorporation of donor species was not a function of the growth interruption time.     

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.     

Ti/Al/Ti/Au and V/Al/V/Au Contacts to Plasma-Etched n-Al0.58Ga0.42N

Cross-sectional transmission electron microscopy was used to study annealed Ti/Al/Ti/Au and V/Al/V/Au ohmic contacts to as-received and plasma-etched n-Al_0.58Ga_0.42N. The reaction depth of low-resistance V-based contacts to as-received n-Al_0.58Ga_0.42N is very limited, unlike previously reported Ti-based contacts to n-Al _x Ga_1−x N. In the present study, the Ti/Al/Ti/Au contacts to as-received n-Al_0.58Ga_0.42N required much higher annealing temperatures than the V-based contacts and also exhibited deeper reactions on the␣order of 40 nm. To achieve a low contact resistance on plasma-etched n-Al_0.58Ga_0.42N, different metal layer thicknesses and processing conditions were required. The Ti- and V-based contacts to plasma-etched n-Al_0.58Ga_0.42N exhibited both similar contact resistances and limited reaction depths, along with the presence of an aluminum nitride layer at the metallization/semiconductor interface. Metal channels penetrate the aluminum nitride layer connecting the top of the metallization to the n-Al_0.58Ga_0.42N. The similarity in phase formation in the contacts to plasma-etched n-Al_0.58Ga_0.42N is likely the reason behind the similarity in specific contact resistances.     

Radiation damage and annealing behaviour in Be+-implanted pure Ga(Al)Sb layers

Implantation of Be⁺ ions into GaAISb epilayers is used to realize thep ⁺ layer of the Ga_0.96Al_0.04Sb p⁺/Ga_0.96Al_0.04Sbn ⁻ /GaSbn ⁺ (1.55 /Μm) avalanche photodetector whose performances are detailed in Ref. (1). The GaAISb layers are grown using liquid phase epitaxy (LPE); the quality of these as-grown layers is shown through photoluminescence and channeling measurements. The last part of this paper is devoted to the damaging level in the Be⁺-implanted layers. Some annealing techniques are presented as a mean of restoration of the implanted layers. It is clear from the results that the Be⁺ ion implantation leads to a low damage level in this III-V compound.     

Photoluminescence characterization of AlGaN-GaN pseudomorphic quantum wells and calculation of strain induced bandgap shifts

The low temperature (77 K) photoluminescence characteristics of Al _x Ga_1-x N-GaN strained layer quantum wells with differentx values grown by metalorganic chemical vapor deposition (MOCVD) were investigated. The photoluminescence spectra were useful in analyzing both quantum confinement effects and strain induced energy shifts. The strain induced shifts were found to be a strong function of aluminum compositionx. A model was developed to calculate the strain induced bandgap shifts atk = 0. The values predicted by this model which took into account the wurtzite crystal structure of the material system, were in good agreement with (i.e. within 2 meV of) the experimentally measured shifts.