A-oriented Substrates Research Articles

Photoluminescence of Europium in ZnO and ZnMgO thin films grown by Molecular Beam Epitaxy

In this work, luminescence properties of Europium doped ZnO and ZnMgO thin films grown on a-oriented (11–20) sapphire substrates by oxygen plasma assisted Molecular Beam Epitaxy method are studied. The concentrations and distributions of Eu atoms in the respective hosts were analysed using Secondary Ion Mass Spectrometry, which confirmed successful doping around the level of ∼1%. Optical properties of the samples were investigated by room temperature photoluminescence (PL) and Photoluminescence excitation (PLE) measurements. The PL data suggest the co-existence of 2+ and 3+ ionic states of Eu in the as-grown samples. It has been observed that alloying Mg with ZnO increases the intensity of the 5D0→7FJ intra-4f-shell emission of Eu3+ ions by an order of magnitude even in as-grown samples. The samples were also annealed at 800 °C in oxygen flux and re-measured to see changes in the PL. After annealing, all the emission lines due to the 5D0→7FJ intra-4f-shell transitions of Eu3+ were strongly enhanced, whereas the bands attributed to Eu2+ were found to diminish or disappear completely. The results of PLE measurements point to efficient host to dopant energy transfer in the studied films.

Structural analysis of the ZnO/MgO superlattices on a-polar ZnO substrates grown by MBE

The analysis of heteroepitaxially grown ZnO/MgO superlattices on a-oriented ZnO substrates was described. The influence of unalike natural crystalline structures of ZnO and MgO on the interface’s abruptness and retaining wurtzite crystallinity was studied. Wurtzite structure was found to be retained in the MgO thin barriers up to 1.5 nm, but creation of nonuniform biaxial strains and some interdiffusion of Mg from barriers to ZnO QW’s seemed to be inevitable. The complex strain/composition heterogeneities can be explained by the difference of vapor pressure between Mg (p = 1.93 Pa) and Zn (p = 0.22 Pa) elements at growth temperatures and high mobility of Mg atoms, which are gladly incorporated into ZnO matrix. As a result, a subtle three monolayers in size ZnMgO film was created at the interfaces, which was partly responsible for retaining the wurtzite structure of coherently grown MgO layers.

Nonpolar short-period ZnO/MgO superlattices: Radiative excitons analysis

The basic structural and detailed analysis of the optical properties of homoepitaxially grown ZnO/MgO superlattices (SLs) on a-oriented ZnO substrates, containing 30 periods of layers, was described. We found using Rutherford backscattering spectroscopy that the wurtzite coordination collapses in the SLs with 3 nm MgO thick barriers. This novel information allows taking this value as the limit for the maximum MgO barrier thickness in such structures. The observed luminescence and the performed calculations of excitonic transitions suggest that such a high energy barrier of MgO allows treating the ZnO layers in these SLs as separate, non-interacting quantum wells. Cross-sectional SEM-CL line scans revealed MgO layer thickness-dependent biaxial strains built up in the QWs structures.

Silicon-Doped Epitaxial Films Grown on GaAs(110) Substrates: the Surface Morphology, Electrical Characteristics, and Photoluminescence Spectra

The results of studies of the surface morphology, electrical characteristics, and photoluminescence properties of epitaxial GaAs films grown by molecular-beam epitaxy on GaAs(110) substrates and doped with Si are reported. A series of samples is grown at a temperature of 580°C with the arsenic/gallium flow ratio in the range from 14 to 80. By analyzing the photoluminescence spectra of the samples, the behavior of Si atoms in GaAs is interpreted with consideration for the occupation of Ga or As sites by Si atoms (i.e., for the formation of SiGa and SiAs point defects) and the formation of vacancies of arsenic and gallium VAs and VGa. In the analysis, the photoluminescence spectra of the samples on (110)-oriented substrates are compared with the photoluminescence spectra of similar samples on (100)- and (111)A-oriented substrates.

THz Radiation of Photoconductive Antennas based on {LT-GaAa/GaAa:Si} Superlattice Structures

A material in the form of a multilayer structure based on low-temperature LT-GaAs grown on (111)A-oriented substrates is proposed for fabrication of THz photoconductive antennas. These structures contain active LT-GaAs layers and doping acceptor GaAs:Si-based layers. At the optical pump power of 19 mW and the bias voltage of 30 V, a photoconductive antenna based on the optimized {LT-GaAs/GaAs:Si} (111)A structure emits THz pulses with the average power of 2.3 μW at the pulse repetition frequency of 80 MHz; the conversion efficiency is 1.2 × 10–4. It is shown that the dependence of the integral power of THz pulses of the antenna based on the {LT-GaAs/GaAs:Si} (111)A structure on the applied voltage is superlinear; the dependence of this parameter on the optical pump power is plotted as a curve with saturation. It is shown that the designed antennas have a practical application in THz spectroscopy of biological solutions.

Photoluminescence study of self-assembled GaAs quantum wires on (631)A-oriented GaAs substrates

The authors report a low-temperature photoluminescence (PL) study of multiple GaAs layers grown between AlAs(0.6 nm)/GaAs(0.6 nm) short-period superlattice barriers (SLBs) simultaneously grown on both GaAs(631)A and (100) substrates. Five GaAs-layers of different nominal thicknesses (LW, ranging from 12 to 2.4 nm) were grown by molecular beam epitaxy. By using (631)A-oriented substrates a self-organized and highly ordered corrugation is obtained in the growth of the GaAs layers, and at the end of the SLB growth, flat surfaces are found. Whereas, for the (100)-oriented sample, flat interfaces are confirmed after the growth of GaAs and SLB layers. By reducing LW below ∼3.6 nm in the (631) sample, strong quantum wire (QWR)-like confinement is achieved as deduced from polarized PL spectroscopy where polarization degrees as large as 0.43 are obtained. The PL emission energy of the (631)-QWRs is redshifted, as compared with the transitions of the (100)-oriented quantum wells, when LW is reduced. The authors explain this energy shift by the widening of the effective thickness of the confinement regions in the GaAs layers.

Structural atomic-scale analysis of GaAs/AlGaAs quantum wires and quantum dots grown by droplet epitaxy on a (311)A substrate

We report the structural analysis at the atomic scale of GaAs/AlGaAs quantum wires and quantum dots grown by droplet epitaxy on a (311)A-oriented substrate. The shape, interfaces, and composition of these nanostructures and their surrounding matrix are investigated. We show that quantum wires can be created by annealing uncapped quantum dots. Substantial interface fluctuations, attributed to interface instability induced by the liquid Ga droplet, are observed. Despite the interface fluctuations, no intermixing of Al was found in either the quantum wires or quantum dots. A wetting layer connecting the quantum dots could not be observed.

As-pressure influence on the surface corrugation in the homoepitaxial growth of GaAs (6 3 1)A

The achievement of defect-free and highly uniform semiconductor quantum wires is a projected goal with many potential applications. In this article, we report on the homoepitaxy of GaAs on (6 3 1)A-oriented substrates grown by molecular beam epitaxy (MBE) as a function of the As 4 pressure ( P As). By finding the optimal growth conditions that allow the minimization of intrinsic surface free energy on the substrate and the P As value, which results in the optimal adatoms diffusion, we were able to realize the outstanding formation of a periodic array of parallel straight nano facets. An analysis of the autocorrelation function is presented, which can be used to quantitatively describe the periodic surface corrugation, and to investigate the optimal growth conditions. We review the thermodynamic and kinetic factors that contribute to the faceting process and discuss how, by reducing the kinetic influence in the growth process, we can promote homogeneous faceting on high-index substrates.

Thermal stability of Ti/Pt/Au ohmic contacts for cryogenically cooled InP-based HEMTs on (4 1 1)A-oriented substrates by MBE

We investigated the contact resistance of a non-alloyed Ti/Pt/Au ohmic electrode to obtain thermally stable source–drain resistance of cryogenically cooled In 0.75 Ga 0.25 As / In 0.52 Al 0.48 As high electron mobility transistors (HEMTs) fabricated on (4 1 1)A-oriented InP substrates by molecular beam epitaxy. A contact resistance of 0.26 Ω mm , which strongly depended on InAlAs spacer and barrier layers, was achieved at 16 K (25% below the value at 300 K) for the non-alloyed Ti/Pt/Au ohmic electrode formed on the In 0.75 Ga 0.25 As / In 0.52 Al 0.48 As HEMT structure with 3-nm-thick InAlAs spacer and 10-nm-thick InAlAs barrier layers. For a 195-nm-gate HEMT, we achieved a maximum transconductance ( g m ) of 2.25 S / mm at 16 K (26% above the value at 300 K), which, to our knowledge, is one of the highest values for HEMTs ever reported. This extremely high g m was attributed to not only 33% lower source–drain resistance ( 0.18 Ω mm at 16 K) because of thermally stable and low contact resistance but also 19% lower transit time under the gate (0.39 ps at 16 K) because of phonon scattering suppression compared with the 300 K values.

Characterization of Stacking Fault-Induced Behavior in 4H-SiC p-i-n Diodes

Formation of I1 Shockley stacking faults by recombination-enhanced defect glide in 4HSiC p-i-n diodes subject to high forward current stress is studied in diodes on both c-oriented and aoriented substrates. The forward voltage increases during stressing for both orientations, accompanied by nucleation and expansion of faults visible in electroluminescence (EL) imaging. Low temperature photoluminescence (PL) measurements on degraded diodes of both orientations reveal the same set of exciton peaks, confirming that the electronic structure of the faults is the same in both cases. The spectroscopic data are compared to self-consistent solutions of the Schrödinger and Poisson equations including polarization charge. Dislocations nucleating the faults are bright in EL images but dark in electron beam-induced current (EBIC) imaging, confirming that they are sites of enhanced radiative recombination.

Optical properties of GaAs/AlAs superlattices grown on (311)A GaAs surfaces

We present a temperature-dependence photoluminescence of (GaAs)5/(AlAs)5 superlattice grown on (311)A-oriented semi-insulating substrate by molecular beam epitaxy. The temperature dependence reveals an anomalous decrease of the PL width, which is explained in terms of phonon-assisted thermal activation of localized excitons.

Lasing characteristics and modal gain of a lateral-junction InGaAs/GaAs edge-emitting laser diode grown on a patterned GaAs (311) A-oriented substrate

Edge-emitting InGaAs/GaAs laser diodes were grown on patterned GaAs (311)A-oriented substrates. Due to the amphoteric properties of Si as a dopant in high-index GaAs, a device with a lateral p–n junction was obtained. CW lasing was observed up to 200 K. The dependence of the threshold current with temperature was measured. The characteristic temperatures were T0=180 and T0=57 K for experimental temperatures below and above 100 K, respectively, meaning that the carrier confinement structure must be improved. Quantitative results of the cavity gain were experimentally obtained.

Lateral p–n junctions for high-density LED arrays

A light-emitting diode array was fabricated using a lateral p–n junction to inject carriers in the InGaAs active layer. The lateral p–n junction is formed in GaAs epilayers doped only with silicon and grown by molecular beam epitaxy on a patterned GaAs (311)A-oriented substrate. This design allows the use of electrically insulating carrier-confining barriers and coplanar contacts while simplifying device process. Light emission is uniform all over the 1200dpi array and electroluminescence spectrum has a single peak at 960 nm.

Gas source molecular-beam epitaxy growth of GaN/GaP superlattices and GaN layers on GaP(1 1 1)A substrates

GaN/GaP short-period superlattices were grown on GaP(1 1 1)A substrates by gas source molecular-beam epitaxy. Despite the large lattice-mismatch between GaN and GaP, reflection high-energy electron diffraction patterns showed the quick switch from GaN (GaP) streaks to GaP (GaN) streaks just after the corresponding switch of the growth. This indicates two-dimensional layer-by-layer growth resulting from the relaxation of lattice strain at the initial stage of the hetero-epitaxy. This phenomenon is considered to be due to the use of (1 1 1)A-oriented substrates. Bulk GaN layers were also grown on GaP(1 1 1)A substrates and the flat-surface GaN layers were obtained because of the two-dimensional layer-by-layer growth starting from the interface between GaN and GaP (1 1 1)A substrates. Strong near-band-edge photoluminescence with very weak deep-level emission was observed. GaP(1 1 1)A substrate is considered to be promising for the growth of GaN.

MOVPE growth of highly-strained piezoelectric InGaAs/GaAs quantum wells on [1 1 1]A-oriented substrates

In this paper we present a study on the MOVPE growth and optical evaluation of strained InGaAs/GaAs quantum well (QW) structures with In contents of 17% and 22% on [111]A-oriented GaAs substrates. QW structures grown under different growth conditions were extensively analyzed by means of Photoluminescence (PL) spectroscopy. The dependence of the PL intensity and linewidth on the growth temperature and In content are investigated and compared to simultaneously grown [100]-oriented structures. It is shown that excellent QW interfaces with only ±(1–2) monolayer roughness were achieved for the [111]A-oriented structures with 17% In using a wide range of growth temperatures (550–670°C), whereas the interfacial properties of the [100] samples significantly degraded as the growth temperature was increased. For a [111]A QW structure with 22% In the PL characteristics are similar to those obtained for the 17% In sample; on the contrary, for the companion [100] sample a strong degradation of its optical properties is observed.

Optical properties of self-assembled InAs quantum dots grown on GaAs(211)A substrate

We have investigated the photoluminescence (PL) and growth properties of self-assembled InAs/GaAs quantum dots (QD) grown on (211)A-oriented GaAs substrate in a low coverage region. At the onset of the QD formation in the Stranski–Krastanov mode, structures of QD on (211)A substrate were quite different from those on (100) substrate. The uniformity of size distribution was better and the density was higher than that grown on (100) substrate. We found a PL peak at 1.32 eV when the InAs coverage was 1.57 ML. Another PL peak gradually appeared at 1.37–1.42 eV with increasing InAs coverage. The peaks at 1.32 and 1.37–1.42 eV were attributed to the emission from a defect-related QD and a typical QD, respectively. When the InAs coverage exceeded 1.89 ML, the QD density decreased with increasing InAs coverage, due to the coalescence of QD. The samples studied here showed PL spectra having a larger intensity and narrower full width at half-maximum compared with that grown on (100) substrate.

Optoelectronic devices based on lateral p-n junctions fabricated by molecular-beam epitaxy growth of silicon-doped GaAs on patterned (311)A-oriented substrates

We demonstrate light-emitting diodes, a vertical-cavity surface-emitting laser (VCSEL), and a photodiode fabricated using a lateral p-n junction. The lateral p-n junction is formed in a GaAs-silicon doped layer grown by molecular-beam epitaxy on a patterned GaAs (311)A-oriented substrate. Lateral p-n junctions have particular properties (i.e., small junction area, coplanar contact geometry, can be clad between electrically insulating layers, allow carrier transport in the plane of multilayer structures, etc.) that are promising for application in new devices. Light-emitting diodes exhibit good electroluminescence at room temperature for both GaAs single layers and GaAs-AlGaAs multiple-quantum-well structures. The VCSEL has electrically insulating distributed Bragg reflectors and coplanar contacts which simplify the device fabrication process. Pulsed-mode operation at room temperature was obtained with a threshold current of 2.3 mA. The light-emission spectrum has a single peak at 942 nm with a full-width at half-maximum of 0.15 nm. The photodiode design allows a reduction of the junction capacitance and an increase of the response speed. A nonoptimized device exhibited a time constant of 10 ps.

Lateral-junction vertical-cavity surface-emitting laser grown by molecular-beam epitaxy on a GaAs (311) A-oriented substrate

A vertical-cavity surface-emitting laser was fabricated using a lateral p-n junction to inject carriers in the InGaAs active layer. The lateral p-n junction is formed in GaAs epilayers doped only with silicon and grown by molecular-beam epitaxy on a patterned GaAs (311) A-oriented substrate. This design allows the use of electrically insulating distributed Bragg reflectors and coplanar contacts while simplifying device process. Pulsed-mode operation at room temperature was obtained with a threshold current of 2.3 mA. Light emission spectrum has a single peak at 942 nm with a full width at half maximum of 0.15 nm.

AlAs oxidation process in GaAs/AlGaAs/AlAs heterostructures grown by molecular beam epitaxy on GaAs (n11)A substrates

The lateral oxidation of AlAs layers grown on GaAs (100), (110) and (n11)A-oriented substrates (n=1, 2, 3, 4) was studied. The temperature dependence of the oxidation rate was measured between 390°C and 450°C. The oxidation rate is highly anisotropic and the anisotropy is related to the symmetry of the crystal structure. The oxidation process has an activation energy that depends on substrate orientation. The oxidation front line becomes irregular for temperatures higher than 450°C and the surface of the samples was degraded when the temperature exceeded 540°C. The time dependence of the oxidation rate was found to be similar to the Si oxidation process.

Lateral wet oxidation of AlAs layer in GaAs/AlAs heterostructures grown by MBE on GaAs (n 1 1)A substrates

Lateral sidewall oxidation of AlAs layers grown on GaAs (1 0 0) and ( n 1 1)A-oriented substrates ( n=1, 2, 3, or 4) within a heterostructure was studied. The oxidation rate for each type of substrate was measured between 390°C and 450°C. The oxidation data concerning the time dependence of the oxidation rate was very well fitted by a theoretical formula based on a Si oxidation model. The time dependence of oxidation rate followed a linear law for the initial process and successively a parabolic law. The oxidation rate is highly anisotropic which is related to the symmetry of the crystal structure. An oxidation model was proposed from the results of anisotropic activation energy of oxidation. Al x O y /GaAs DBRs (distributed Bragg reflectors) were fabricated and their reflection characteristics were measured. It is shown that the center wavelength of DBRs can be adjusted during the oxidation process.