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.     

Optical properties and defects in GaAsN and InGaAsN films and quantum well structures

Photoluminescence and microcathodoluminescence spectra of thick-film GaAsN and InGaAsN structures and GaAs/InGaAsN, AlGaAs/InGaAsN quantum wells (QWs) were studied for InGaAsN layers with low nitrogen concentration of 0.35–0.5%. It is shown that in thick-film structures the bandedge luminescence intensity is strongly decreased in the row homoepitaxial GaAs, GaAsN on GaAs buffer, GaAsN, GaAs on GaAsN buffer, InGaAsN which correlates with the increasing concentration of electron traps with activation energy 0.53–0.55 eV. The type of defect bands in the thick-film structures was found to strongly depend on composition of the layers. For the GaAs/InGaAsN QW structures the intensity of luminescence was found to be more than an order of magnitude higher than in InGaAsN single films.     

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.     

Effects of GaAs buffer layer and lattice-matching on deep levels in Zn(S)Se/GaAs heterostructures

The effects of GaAs buffer layer and lattice-matching on the nature of deep levels involved in Zn(S)Se/GaAs heterostructures are investigated by means of deeplevel transient spectroscopy (DLTS). The heterojunction diodes (HDs) where nZn(S)Se is grown on p⁺-GaAs by metalorganic vapor phase epitaxy are used as a test structure. The DLTS measurement reveals that when ZnSe is directly grown on a GaAs substrate, there exist five electron traps A-E at activation energies of 0.20, 0.23, 0.25, 0.37, and 0.53 eV, respectively. Either GaAs buffer layer and lattice-matching may reduce the incorporation of traps C, D, and E, implying that these traps are ascribed to surface treatment of GaAs substrate and to lattice relaxation. Concentration of trap B, which is the most dominant level, is proportional to the donor concentration. However, in the ZnSSe/GaAs sub. HD, another trap level, instead of trap B, locates at the almost same position as that of trap B, and it shows anomalous behavior that the DLTS peak amplitude changes drastically as changing the rate windows. This is explained by the defect generation through the interaction between sulfide and a GaAs substrate surface. For the trap A, the concentration is a function of donor concentration and lattice mismatch, and the origin is attributed to a complex of donor induced defects and dislocations.     

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.     

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.     

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.     

The growth and properties of mixed group V nitrides

Bearing in mind the problems of finding a lattice-matched substrate for the growth of binary group III nitride films and the detrimental effect of the large activation energy associated with acceptors in GaN, we propose the study of the alloy system AlGaAsN. We predict that it may be possible to obtain a direct gap alloy, with a band gap as wide as 2.8eV, which is lattice-matched to silicon substrates. The paper reports our attempts to grow GaAsN alloy films by molecular beam epitaxy on either GaAs or GaP substrates, using a radio frequency plasma source to supply active nitrogen. Auger electron spectra demonstrate that it is possible to incorporate several tens of percent of nitrogen into GaAs films, though x-ray diffraction measurements show that such films contain mixed binary phases rather than true alloys. An interesting observation concerns the fact that it is possible to control the crystal structure of GaN films by the application of an As flux during growth. In films grown at 620°C a high As flux tends to increase the proportion of cubic GaN while also resulting in the incorporation of GaAs. Films grown at 700°C show no evidence for GaAs incorporation; at this temperature, it is possible to grow either purely cubic or purely hexagonal GaN depending on the presence or absence of the As beam.     

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.     

Oxygen behavior in liquid phase epitaxial GaAs

The oxygen activity in the gallium solvent was controlled and measured by an electrochemical method, during the liquid phase epitaxy of GaAs. This technique, in combination with SIMS analysis of GaAs grown from O¹⁸ saturated melts, was used to determine the oxygen distribution coefficient at 840°C. In this temperature region, the low solubility of oxygen in liquid gallium combined with a distribution coefficient ? 10^?3 leads to maximum oxygen levels in the ? 10¹⁶ cm^?3 range. The behavior of oxygen in three layers, with significant differences in oxygen concentration, was evaluated by photoluminescence and deep level transient spectroscopy, and no direct relationship was observed between the energy levels in the PL or DLTS spectra and the oxygen concentration in epitaxial GaAs.     

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.     

Neutron-induced trapping levels in aluminum gallium arsenide

Deep level transient spectroscopy (DLTS) measurements have been performed on a variety of Al_xGa_1-xAs p-n junctions prior to and following a series of fast neutron irradiations at room temperature and subsequent isochronal anneals. In contrast with electron and proton irradiated GaAs, neutron irradiation produces a single, broad featureless DLTS band which is a majority carrier trap in both n and p type material. The characteristics of this neutron-induced trap are relatively independent of growth method, dopant type and concentration. In GaAs, the thermal emission energies of the trap are 0.58 to 0.68 eV depending on the particular junction. These energies increase with Al content to 0.94 eV at 20% Al. The trap introduction rate, which also increases with Al content, is 0.7 cm_-1 in GaAs. Isochronal annealing to temperatures as high as 400‡C results in a smaller FWHM of the DLTS band, a shift in the peak to higher temperatures, and a modest decrease in magnitude. Above 400‡C the magnitude decreases rapidly, suggesting a similarity with the antisite defect, As_Ga, which has been observed to anneal in this range.     

Effects of carrier confinement by InGaAs/GaAs heterointerfacebarrier on deep trap concentration profiling

Carrier confinement by the conduction-band barrier formed by the band discontinuity ΔE_c has been observed in molecular beam epitaxy (MBE)-grown In_0.1Ga_0.9As/GaAs heterostructures as evidenced by the decrease of DLTS (deep level transient spectroscopy) peak height with increased filling pulse amplitude in a fixed-reverse-bias variable-filling-pulse DLTS study. A DLTS model including the ΔE_c effects on trap profiling is developed by considering conduction-band barrier as a giant deep trap with an electron emission rate depending exponentially on ΔE_c. The model explains quantitatively the experimental observations of the effects of ΔE_c in the DLTS study and shows that an erroneous trap concentration profile may result from the conduction-band barrier if the standard DLTS model is used. It is shown that a 0.16-eV±0.01-eV electron trap reported to be located only at the heterointerface of In_0.53Ga_0.47As/InP may actually be a deep-trap characteristic of InGaAs with a fairly uniform trap concentration throughout the InGaAs epilayer     

溅射ZnO薄膜钝化GaAs表面性能的研究

为了改善GaAs(110)与自身 氧化物界面由于高表面态密度而引起的费米能级钉扎(pinning)问题 ,提出采用射频磁控溅射技 术在GaAs(110)衬底上沉积一定厚度 ZnO薄膜作为钝化层,并利用光 致发光(PL)光谱和X射线光电子能谱(XPS) 等方法对ZnO薄膜的光学特性及钝化性能进行表征。实验结果表明,经ZnO薄膜钝化后的 GaAs样品,其本征PL峰强度提高112.5%,杂质峰强度下降82.4%。XPS光谱分析表明,Ga和As原子的比值从1.47降低 到0.94,ZnO钝化层能 够抑制Ga和As的氧化物形成。因此,在GaAs表面沉积ZnO薄膜是一种可行的GaAs表面钝化 方法。     

Structural properties of GaAsN grown on (001) GaAs by metalorganic molecular beam epitaxy

Detailed transmission electron microscopy (TEM) and transmission electron diffraction (TED) examination has been made of metalorganic molecular beam epitaxial GaAsN layers grown on (001) GaAs substrates. TEM results show that lateral composition modulation occurs in the GaAs_1−xN_x layer (x 6.75%). It is shown that increasing N composition and Se (dopant) concentration leads to poor crystallinity. It is also shown that the addition of Se increases N composition. Atomic force microscopy (AFM) results show that the surfaces of the samples experience a morphological change from faceting to islanding, as the N composition and Se concentration increase. Based on the TEM and AFM results, a simple model is given to explain the formation of the lateral composition modulation.     

微重力条件下生长GaAs单晶的电学和光学性质研究

用多种实验手段分别对地面和太空生长的掺Te砷化镓单晶的电学、光学均匀性和深能级行为进行了实验研究.初步结果表明:在太空进行再生长的GaAs单晶电子浓度比原地面生长的籽晶小一个数量级,电子浓度由地面晶体到太空晶体的过渡是陡变的;DLTS测量发现太空单晶中存在两个电子陷阱,分别位于导带下0.27eV和0.60eV处,深能级密度为浅施主N_D的10~(-3)-10~(-4);少子注入未观察到空穴陷阱;用太空GaAs单晶为衬底制备的25个单异质结(SH)二极管,具有一致的I-V和发光特性,这反映了太空晶体的均匀性优于地面晶体.此外,还对太空生长GaAs单晶电子浓度降低的可能原因、深能级行为以及太空生长高质量晶体的前景作讨论.     

Electrically Active Defects in GaN Layers Grown With and Without Fe-doped Buffers by Metal-organic Chemical Vapor Deposition

Electrically active defects in n-GaN films grown with and without an Fe-doped buffer layer have been investigated using conventional and optical deep-level transient spectroscopy (DLTS). Conventional DLTS revealed three well- defined electron traps with activation energies E _a of 0.21, 0.53, and 0.8 eV. The concentration of the 0.21 and 0.8 eV defects was found to be slightly higher in the sample without the Fe-doped buffer, whereas the concentration of the 0.53 eV trap was higher in the sample with the Fe-doped buffer. A minority carrier trap with E _a ≈ 0.65 eV was detected in both samples using optical DLTS; its concentration was ∼40% higher in the sample without the Fe-doped buffer. Mobility spectrum analysis and multiple magnetic-field measurements revealed that the electron mobility in the topmost layer of both samples was similar, but that the sample without the Fe-doped buffer layer was affected by parallel conduction through underlying layers with lower electron mobility.     

热壁外延生长GaAs／Si薄膜质量研究

本文研究了用热壁外延（HWE）技术在Si衬底上不同工艺下生长的GaAs薄膜的拉曼（Raman)和荧光（PL）光谱。研究表明：在室温下,GaAs晶膜的Raman光谱的265cm^-1横声子（TO）峰和289cm^-1纵声子（LO）峰的峰值之比随晶膜质量的变化而逐渐变大、半高宽（FWHM）变窄且峰值频移动变小,而LC光谱出现在871nm光谱的FWHM较窄,表明所测得的薄膜为单晶晶膜,对同一晶膜也可判断出均匀程度。因此可以通过拉曼光谱和PLC光谱相结合评定外延膜晶体质量。     

Electron traps created by high temperature annealing in MBE n-GaAs

An electron trap with a thermal activation energy of 0.83 eV from the conduction band is common in the deep level transient spectroscopy (DLTS) spectra of vapor phase epitaxial (VPE) n-GaAs, but is not observed in the DLTS spectra of as-grown molecular beam epitaxial (MBE) n-GaAs. We show here that this trap is created during high temperature annealing of MBE samples with a Si₃N₄, encapsulant. The trap concentration is correlated with the annealing temperature and time, suggesting the outdiffusion of a constituent atom resulting in the formation of a vacancy or vacancy-complex. Other electron traps observed in the DLTS spectra of asgrown MBE n-GaAs are annealed out for temperatures at or above 800° C.     

Nanometer-scale studies of nitride/arsenide heterostructures produced by nitrogen plasma exposure of GaAs

We have investigated the nanometer-scale structure and electronic properties of nitride/arsenide superlattices produced by nitridation of a molecular beam epitaxially grown GaAs surface. Using cross-sectional scanning tunneling microscopy and spectroscopy, we find that the nitrided layers are not continuous films, but consist of groups of atomic-scale defects and larger clusters. We identify the defects and clusters as N_As and GaN with dilute As concentration, respectively. Thus, the nitrided regions consist of alloys from both sides of the miscibility gap predicted for the GaAsN system. In addition, spectroscopy on the clusters reveals an upward shift of the band edges and band gap narrowing, with significant change in the conduction band structure. We estimate the contribution of strain to band gap narrowing in terms of an elasticity calculation for a coherently strained spherical GaN cluster embedded in GaAs.