Studies of deep centers in dilute GaAsN and InGaAsN films grown by molecular beam epitaxy

Deep levels spectra DLTS, 77 K photoluminescence (PL) spectra and photosensitivity were measured for GaAsN and InGaAsN films with low N and In concentration grown by molecular beam epitaxy and in GaAs films grown on GaAsN buffer. It is shown that the bandedge luminescence intensity is greatly decreased in GaAsN, GaAs/GaAsN and particularly in InGaAsN structures compared to the homoepitaxial GaAs. Comparison of the DLTS and PL spectra strongly suggests that the main recombination center in such films is the EL3-like electron trap whose concentration greatly increases upon In and N incorporation into the solid solution. Based on published results the trap is associated with substitutional oxygen on As site and the results are discussed in view of such possible assignment.     

Interface properties and deep levels in InGaAsN/GaAs and GaAsN/GaAs heterojunctions

Interface properties of dilute slightly lattice mismatched GaAsN/GaAs (0.35 at.% N) and closely lattice matched InGaAsN (1 at.% In, 0.35 at.% N) heterojunctions (HJs) were studied by means of capacitance–voltage profiling, deep levels transient spectroscopy (DLTS) and current–voltage measurements. It is found that the lattice matched HJs show no electrical breakdown when the space charge region crosses the interface. The carrier concentration profiles in such HJ show, as expected, the accumulation region on the low-bandgap side and the depletion region on the high-bandgap side of the HJ. This is not the case for the GaAsN/GaAs (GaAsN layer on top) and the GaAs/GaAsN (GaAs layer on top) HJ. The density of deep traps in GaAsN, InGaAsN films and in GaAs films grown on GaAsN underlayers was very much higher than in epitaxial GaAs films. The dominant deep centers were the EL6 and the EL3 electron traps. The interface regions of the GaAs/GaAsN and the InGaAsN/GaAs HJs were shown to be enriched by EL3 traps, while for the GaAsN/GaAs HJ those regions were enriched by EL6 traps which was associated with the former films being Ga-rich and thus facilitating incorporation of oxygen on As sites.     

Methods of controlling the emission wavelength in InAs/GaAsN/InGaAsN heterostructures on GaAs substrates

Studies of the properties of InGaAsN compounds and methods of controlling the emission wavelength in InAs/GaAsN/InGaAsN heterostructures grown by molecular beam epitaxy on GaAs substrates are reviewed. The results for different types of heterostructures with quantum-size InGaAsN layers are presented. Among those are (1) traditional InGaAsN quantum wells in a GaAs matrix, (2) InAs quantum dots embedded in an (In)GaAsN layer, and (3) strain-compensated superlattices InAs/GaAsN/InGaAsN with quantum wells and quantum dots. The methods used in the study allow controllable variations in the emission wavelength over the telecommunication range from 1.3 to 1.76 μm at room temperature.     

The optical properties of heterostructures with quantum-confined InGaAsN layers on a GaAs substrate and emitting at 1.3–1.55 μm

Under study is the photoluminescence of two types of MBE-grown heterostructures with quantum-confined InGaAsN/GaAs layers: (1) conventional InGaAsN quantum wells (QWs) in GaAs, and (2) heterostructures with an active region consisting of a short-period GaAsN/InGaAsN superlattice that has an InGaAsN QW with a submonolayer InAs insertion at its center. At room temperature, the structures under study emit light in the range from ～1.3 to ～1.55 μm. In the second type of heterostructure, emission with a wavelength larger than 1.5 μm can be obtained at lower nitrogen and indium concentrations than in a conventional QW. This leads to a significant depression of the effects related to decomposition of an InGaAsN solid solution, thus improving the radiative efficiency of the InGaAsN QWs.     

Long wavelength MOCVD grown InGaAsN-GaAsN quantum well lasers emitting at 1.378-1.41 /spl mu/m

Low threshold InGaAsN QW lasers with lasing wavelength at 1.378 and 1.41 /spl mu/m were demonstrated by metal organic chemical vapour deposition (MOCVD). The threshold current densities are 563 and 1930 A/cm/sup 2/ for the 1.378 and 1.41 /spl mu/m emitting lasers, respectively. The significant improvement of device performance is believed due to utilisation of high temperature annealing and introduction of GaAsN barriers to suppress the resulting wavelength blue shift. A comparable characteristic temperature coefficient of the external differential quantum efficiency, T/sub 1/, is observed for the InGaAsN-GaAsN QW laser compared to similar InGaAsN/GaAs structures.     

GaAs/InGaAsN heterostructures for multi-junction solar cells

Solar-cell heterostructures based on GaAs/InGaAsN materials with an InAs/GaAsN superlattice, grown by molecular beam epitaxy, are studied. A p-GaAs/i-(InAs/GaAsN)/n-GaAs p–i–n test solar cell with a 0.9-μm-thick InGaAsN layer has an open-circuit voltage of 0.4 V (1 sun, AM1.5G) and a quantum efficiency of >0.75 at a wavelength of 940 nm (at zero reflection loss), which corresponds to a short-circuit current of 26.58 mA/cm² (AM1.5G, 100 mW/cm²). The high open-circuit voltage demonstrates that InGaAsN can be used as a material with a band gap of 1 eV in four-cascade solar cells.     

Optical properties of MBE-grown ultrathin GaAsN insertions in GaAs matrix

Optical properties of MBE-grown ultrathin GaAsN insertions in a GaAs matrix have been studied, with the goal of deriving methods for the intentional formation of carrier localization regions in GaAsN layers. In the case of a short-period GaAs/GaAsN superlattice, an additional long-wavelength line is observed in the photoluminescence spectrum. Comparison of the optical data with the transmission electron microscopy data shows that this line is related to the emission from the formed regions enriched with nitrogen (up to 8.5%). The characteristic size of these nitrogen-enriched regions in upper layers of the superlattice is larger than in lower ones, and it increases with the number of the deposited layers increasing.     

Structural and optical properties of heterostructures with InAs quantum dots in an InGaAsN quantum well grown by molecular-beam epitaxy

Results obtained in a study of the structural and optical properties of GaAs-based heterostructures with InAs quantum dot layers overgrown with InGaAsN quantum wells are presented. Transmission electron microscopy has been applied to analyze how the thickness of the InGaAsN layer and the content and distribution of nitrogen in this layer affect the size of nanoinclusions and the nature and density of structural defects. It is shown that the size of InAs nanodomains and the magnitude of the lattice mismatch in structures containing nitrogen exceed those in nitrogen-free structures. A correlation between the luminescence wavelength and the size and composition of nanodomains is demonstrated. Furthermore, a correlation between the emission intensity and defect density in the structure is revealed.     

Band-edge line-up in GaAs/GaAsN/InGaAs heterostructures

Triple GaAs/GaAsN/InGaAs heterostructures were grown by MBE on GaAs substrates, and their optical properties were studied. The band-edge line-up in GaAs/GaAsN and InGaAs/GaAsN heterostructures was analyzed by correlating the experimental photoluminescence spectra with the known parameters of the band diagram in (In,Ga)As compounds. It is shown that a GaAs/GaAsN heterojunction is type I, while an In_xGa_1?x As/GaAsN heterojunction can be type I or type II, depending on the In content x.     

Effect of uniform compression on photoluminescence spectra of GaAs layers heavily doped with beryllium

The effect of high-temperature annealing at high hydrostatic pressures on photoluminescence of heavily doped GaAs:Be layers grown by molecular-beam epitaxy on GaAs substrates was studied. A blue shift of the band-edge luminescence line and an increase in the relative intensity of the shoulder at the high-energy wing of this line were detected after annealing in the spectra of layers with a beryllium atom concentration higher than 5×10¹⁹ cm^?3. The same layers featured a concentration-related decrease in the GaAs lattice parameter, which does not conform to the Vegard law. These effects can be attributed to the formation of beryllium inclusions in heavily doped GaAs. Due to different compressibility and thermal expansion coefficients of Be inclusions and GaAs, high-temperature and high-pressure treatment gives rise to structural defects; hence, the probability of transitions that are indirect in the k space increases.     

Experimental observation of splitting of the light and heavy hole bands in elastically strained GaAsN

Twin peak photoluminescence of a GaAsN solid solution grown on GaAs substrate has been observed at room temperature. The peak splitting increases with an increase in the nitrogen content of the ternary compound. The observed form of the spectra is attributed to the presence of two transitions involving light and heavy holes. The splitting of light-and heavy-hole levels is due to elastic strain in GaAsN layers grown on the GaAs surface.     

Effect of carrier localization on the optical properties of MBE-grown GaAsN/GaAs heterostructures

The optical properties of GaAsN/GaAs heterostructures grown by molecular-beam epitaxy with different nitrogen content in the layers have been studied. The optical properties of GaAsN layers in the growth conditions under study are defined by the carrier recombination via localized states related to a strong composition inhomogeneity in the solid solution. The increasing of the nitrogen content raises the composition inhomogeneity and the carrier localization energy.     

Incorporation behaviour of carbon and silicon on (100) and (111) surfaces during growth of GaAs/GaAs and Ga0.47In0.53As/InP by molecular beam epitaxy

Silicon and carbontetrabromide were used as dopant sources in the growth of GaAs/GaAs and Ga_0.47In_0.53As/InP structures. We studied the incorporation behaviour of these group IV atoms on (100) and {111} surfaces as a function of growth temperature. The free carver concentrations determined by Hall measurements for Si-doped GaAs and Ga_0.47In_0.53As layers are independent of growth temperature on all surface orientations studied. Silicon acts fundamentally as a donor except, as expected, for doped layers on (111)A GaAs substrates, where it acts as an acceptor. Carbon incorporation in GaAs and Ga_0.47In_0.53As always results in a p-type conduction independent of the surface orientations (100)/{111} or the growth temperatures we used. In contrast to the results on GaAs, carbon shows a strong temperature-dependent activation in Ga_0.47In_0.53As grown on (100) and (111)B surfaces. Carbon-doped Ga_0.47In_0.53As on (111)A and carbon-doped GaAs layers on (100)/{111} GaAs surfaces exhibit only a very weak dependence of the carrier concentration on the growth temperature. A significant amphoteric behaviour of carbon was not observed in any of the materials investigated.     

Magnetic field effects in the luminescence spectra of type II GaAs/AlAs double layer structures

We report on low temperature luminescence studies of type II GaAs/AlAs heterostructures in magnetic fields up to 20 T. Application of magnetic fields perpendicular to the two-dimensional planes lead to a tremendous quenching of the indirect luminescence transitions. At higher excitation intensities, when the luminescence transitions are of a more Γ-Γ character within the GaAs layer, the application of magnetic field leads to an increase of the luminescence intensity. These effects are attributed to field induced lateral localization of photocreated carriers.     

Annealing behavior of InAs/GaAs quantum dot structures

We investigate the annealing behavior of InAs layers with different thicknesses in a GaAs matrix. The diffusion enhancement by strain, which is well established in strained quantum wells, occurs in InAs/GaAs quantum dots (QDs). A shift of the QD luminescence peak toward higher energies results from this enhanced diffusion. In the case of structures where a significant portion of the strain is relaxed by dislocations, the interdiffusion becomes negligible, and there is a propensity to generate additional dislocations. This results in a decrease of the QD luminescence intensity, and the QD peak energy is weakly affected.     

自组织生长InAs／GaAs量子点的退火效应

通过研究ＧａＡｓ衬底上不同厚度ＩｎＡｓ层光致发光的退火效应，发现它和应变量子阱结构退火效应相类似，ＩｎＡｓ量子点中的应变使退火引起的互扩散加强，量子点发光峰蓝移．量子点中或其附近一旦形成位错，其中的应变得到释放，互扩散现象就不明显了，退火倾向于产生更多的位错，量子点的发光峰位置不变，但强度减弱．     

Realization of one-chip-two-wavelength light sources

We proposed a vertically integrated one-chip-two-wavelength light source which consists of a separate confinement single-quantum-well (SCH-SQW) ZnCdSe/ZnSe/ZnMgBeSe heterostructure for blue–green light emitters grown on SCH-MQW InGaP/InGaAlP for red light-emitting devices.We investigated, firstly, the effect of a thin low-temperature-grown ZnSe buffer layers (LT-ZnSe) in improving ZnSe crystallinity by inserting it between the high-temperature-grown ZnSe epilayer and the GaAs substrate, secondly, the growth optimization of LT-ZnSe on tilted GaAs (0 0 1) substrate, and lastly, the molecular beam epitaxy growth and characterization of ZnCdSe/ZnSe/ZnMgBeSe quantum well structures on metal organic chemical vapor deposition (MOCVD) grown III–V red light emitters.Optically pumped lasing is achieved from II–VI and III–V laser structures on one chip at room temperature. The present results clearly show the feasibility of epitaxial integration of II–VI and III–V laser structures.     

Device characteristics of the GaAs/InGaAsN/GaAs p-n-p doubleheterojunction bipolar transistor

We have demonstrated the dc and rf characteristics of a novel p-n-p GaAs/InGaAsN/GaAs double heterojunction bipolar transistor. This device has near ideal current-voltage (I-V) characteristics with a current gain greater than 45. The smaller bandgap energy of the InGaAsN base has led to a device turn-on voltage that is 0.27 V lower than in a comparable p-n-p AlGaAs/GaAs heterojunction bipolar transistor. This device has shown f_T and f_MAX values of 12 GHz. In addition, the aluminum-free emitter structure eliminates issues typically associated with AlGaAs     

Improved performance of submicrometer-gate GaAs MESFETs with an Al0.3Ga0.7As buffer layer grown by metal organicvapor phase epitaxy

Submicrometer-gate MESFETs were fabricated with a GaAs active layer and an Al_xGa_1-xAs buffer layer grown by metalorganic vapor-phase epitaxy. To investigate the effect of buffer layer composition on device performance, microwave FETs with GaAs and Al _0.3Ga_0.7As buffer layers were compared. Electron Hall mobility in the n-GaAs active layer was found to be unaffected by the Al content or carrier concentration in the buffer layer. However, a considerable improvement in the maximum available gain to as much as 5.2 dB was obtained at 26.5 GHz for FETs with a p-Al_0.3Ga_0.7 As buffer layer; this was 1.5 dB higher than the gain obtained with a p-GaAs buffer layer. The improvement is due to a 20-30% reduction in both drain conductance and drain-gate capacitance     

Tunneling and energy relaxation of hot electrons in double quantum well structures

The tunneling and cooling times of photoexcited hot electrons in AlGaAs/GaAs double (one narrow and the other wide) quantum well structures have been measured using photoluminescence excitation correlation spectroscopy. The tunneling times are 180 – 320 ps depending on the structure. The tunneling time assisted by the emission of optical phonon is found to be shorter than that assisted by the emission of acoustic phonons. The cooling time of hot electrons studied as a function of the kinetic energy shows a strong effect of the carrier concentration on the energy loss rate.