group-III Atoms Research Articles

Nitrogen supply rate dependence of InGaN growth properties, by RF-MBE

Nitrogen flow rate dependence of InGaN growth mechanism on sapphire (0 0 0 1) was studied. The indium content of the InGaN layer can be controlled by changing the nitrogen supply rate by structural and optical investigations. In case of lower nitrogen flow rate, the formation of phase separation due to dissociation of InN and condensation of indium on the growing surface were observed. In case of higher nitrogen flow rate (under N-rich condition), on the other hand, the indium content became uniform and the photoluminescence (PL) emission showed single peak. It is considered that the causes of phase separation formation were suppressed and nonequilibrium condition was enhanced by the increase of the nitrogen supply rate and the growth rate. Judging from the estimated thermal activation energies and degree of fluctuation in the indium content by PL line-width, however, the non-radiative recombination centers within the spontaneously formed potential wells as well as in the extended states were formed in case of excess nitrogen supply rate. The structural defects that were caused by the suppression of the surface migration of group-III atoms with excessive activated nitrogen might have worked as non-radiative recombination centers. Thus, a slightly N-rich condition is desirable for the realization of the uniform InGaN alloy composition and the high optical quality, especially at relatively high growth temperature.

Atomic scale analysis of MOVPE grown heterostructures by X-ray crystal truncation rod scattering measurement

The origin of the X-ray CTR scattering have been described in brief in order to make it clear why the X-ray CTR scattering measurement can reveal the interface structure. Then, some examples of the results obtained by the X-ray CTR scattering measurement have been shown. The distributions of group-V and III atoms in InP/GaInAs/InP double-heterostructure samples have mainly been discussed. The change of the distribution of As at the interface with the change of H 2 -purge time has been quantitatively shown at an atomic scale by the X-ray CTR scattering measurement, which has demonstrated the high resolution and significance of the X-ray CTR scattering measurement in the investigation of the III–V compound semiconductor heterostructures. The distribution of group-III atoms has also been investigated by the X-ray CTR scattering measurement. The results have suggested that the interface has not been so abrupt as expected, and it has been more difficult to control the distributions of group-III atoms than group-V atoms. The results have shown how important the observation by the X-ray CTR scattering has been to understand the interface structure.

Annealing effects in InGaAsSb quantum wells with pentenary AlInGaAsSb barriers

We have previously shown that rapid thermal annealing (RTA) of Te-doped GaSb can increase the n-type carrier concentration (Bugge et al 2004 13th Int. Conf. on Mol. Beam Epitaxy (Edinburgh)). In order to see whether this process can be used on a complete laser structure, we have studied the effect of RTA on the active layers and cladding layers of a GaSb-based laser, as well as on the defect structure in the material. Multiple-quantum-well (MQW) structures of InGaAsSb/AlGaAsSb and InGaAsSb/AlInGaAsSb were grown by molecular beam epitaxy (MBE) on GaSb substrates. The MQW structures were examined with photoluminescence (PL) and x-ray diffraction prior to and after RTA at 625°C and at 650°C for 10 s to look for differences in the (QW) structure. Highly Te-doped layers of GaSb grown by MBE were examined with PL to yield some information on the change in defect structure during RTA. The results show that atomic interdiffusion across the QW heterointerfaces plays a major role during RTA and that it can reduce interface strain for InGaAsSb/AlInGaAsSb MQWs. The diffusion length of group-III atoms was found to be about 2 nm after annealing at 650°C for 10 s.

Group-III intermixing in InAs∕InGaAlAs quantum dots-in-well

We report selective postgrowth band gap tuning of InAs∕InGaAlAs quantum dots-in-well grown on InP substrate using impurity-free group-III intermixing. In contrast to most reported intermixing results, SixNy annealing cap results in a larger band gap blueshift than SiO2 annealing cap with a differential shift of 92nm after annealing at 800°C for 30s. Intermixing also results in large wavelength tuning from 1.6to1.37μm at room temperature, accompanied by luminescence linewidth reduction and intensity improvement. According to our theoretical model, we postulate that the unusual In(GaAl)As intermixing is governed by different interdiffusion rates of group-III atoms.

Spatial Bandgap Tuning in Long Wavelength InAs Quantum Dots-in-Well Laser Structure

ABSTRACTWe employed the postgrowth impurity-free vacancy diffusion technique to selectively tune the bandgap of the InAs/InGaAlAs dots-in-well laser structure grown on (100) InP substrate. A blueshift up to 170 nm with a significant decrease in the photoluminescence linewidth has been observed. Spatial control of the bandgap shifts has been achieved using SiO2 and SixNy layers as annealing caps. A differential wavelength shift of 76 nm has been observed after a rapid thermal annealing step at 750 °C for 30 s. In contrast to most reported results in other material systems using similar process, we observed a larger bandgap shift from the SixNy capped samples than from the SiO2 capped samples. Our theoretical calculation indicates that the unusual intermixing behavior in this material system is governed by different interdiffusion rates of group-III atoms.

Molecular beam epitaxy of compound materials through shadow masks

We present a consistent model for growth dynamics in molecular beam epitaxy of compound materials through shadow masks. The model takes into account specific properties of II-VI and III-V material systems. In the case of the II-VI compounds, the redistribution of molecular fluxes under the shadow masks plays a crucial role. Based on the model, we show that reactions between the constituent species determine the flux distribution within the mask cavity, which also affects the deposition of compound material. In contrast, for III-V materials surface diffusion is the key factor responsible for the shape of the deposits. Our model considers also that the diffusing species interacts with the molecular flux of the other constituent. Hence, surface diffusion of group-III atoms is controlled by the influx of group-V species, which in turn is determined by the growth geometry. In addition, we demonstrate that surface diffusion takes place in absence of group-III flux. The formation of the deposits in characteristic shapes, observed in our experiments, can also be explained based on the model.

Atomic layer diffusion and electronic structure at In0.53Ga0.47As/InP interfaces

We have used secondary ion mass spectrometry and cathodoluminescence spectroscopy to determine the effects that growth and postgrowth conditions have on interdiffusion and near band edge emissions in In0.53Ga0.47As/InP heterojunctions grown by molecular beam epitaxy. This lattice-matched interface represents a model system for the study of atomic movements and electronic changes with controlled anion overlap during growth. Structures subjected to anneals ranging from 440 to 495 °C provide a quantitative measure of concentration-driven cross diffusion of group-III and group-V atoms. By measuring anneal-induced broadening at the InGaAs-on-InP interface we have determined an activation energy for As diffusion into InP of ∼2.44±0.40 eV. An interface layer with Ga–P bonds indicates Ga competes favorably versus As for bonding in the preannealed InP near-surface region. In addition, we present evidence that interface chemical effects manifest themselves electronically as variations of the InGaAs band gap energy.

Investigation of IrO2 and RuO2 Schottky contacts on AlGaN/GaN heterostructure

Electrical properties of rare-earth metal contacts on AlGaN/GaN heterostructure were interpreted in terms of the changes in microstructure and chemical bonding state. When the contacts were annealed under oxygen ambient at 500 °C, the Schottky barrier height increased from 0.56 to 1.10 eV for the Ru and from 0.68 to 1.07 eV for the Ir contact. Moreover, the reverse leakage current at −10 V dramatically reduced by 4 orders of magnitude by oxidation annealing. Such an improvement originated from the formation of RuO2 and IrO2, playing a key role in increasing the solubility of group-III atoms, namely, Ga and Al atoms. As a result, the surface Fermi level shifted toward the energy levels of group-III vacancies, resulting in the increase of Schottky barrier height. The electrical properties of heterostructure field effect transistor (HFET) applying the Ru gate contact significantly degraded when the device was annealed at 500 °C under O2 ambient. This was due to the indiffusion of the Ru atoms into the AlGaN layer during the oxidation annealing. However, no electrical degradation was found in the HFET using the Ir gate contact. The maximum drain current density of 714 mA/mm and transconductance of 171 mS/mm were kept even after annealing at 500 °C.

Composition dependence of InP/Ga xIn 1− xAs yP 1− y/InP interface structures analyzed by X-ray CTR scattering measurements

Composition and stress dependences of InP/Ga x In 1− x As y P 1− y /InP interface structures were investigated using X-ray crystal truncation rod (CTR) scattering measurements, in order to realize why group-III atoms were always distributed widely around hetero-interfaces between InP and GaInAs(P). The result showed that the change of the Ga and As compositions to keep the lattice constants and the change of the stress to keep the Ga composition did not affect the degree of the distribution of the group-III atoms. It suggested that the distribution of the group-III atoms was due neither to mixing of In and Ga nor to relaxing local stress at around the interfaces. The distribution was probably related to the growth process at the surface in the organometallic vapor phase epitaxy (OMVPE) system.

Diffusion at the interfaces of InGaNAs/GaAs quantum wells

Interdiffusion of the group-III atoms was interpreted for the structural and optical changes observed in X-ray diffraction and PL spectra of post-growth annealed 1.3-μm Ga 1− x In x N y As 1− y /GaAs material systems. Interdiffusion of As and N was observed to be much less than that of group-III in the temperature range studied in this work. A diffusion barrier, GaInNAs, grown on either side of the quantum well reduced interdiffusion very effectively. As a result, a blue shift of photoluminescence caused by annealing was prevented to a large extent, emission intensity was improved, and the spectra were shifted towards longer wavelengths, due to broader effective quantum wells. It was also found that interfacial stress tended to enhance diffusion.

Far-Infrared dielectric anisotropy and phonon modes in spontaneously CuPt-orderedGa0.52In0.48P

The phonon properties of partially ${\mathrm{CuPt}}_{B}$-ordered ${\mathrm{Ga}}_{0.52}{\mathrm{In}}_{0.48}\mathrm{P}$ with degrees of ordering $\ensuremath{\eta}=0,$$0.19,$$0.28,$$0.35,$$0.47,$ determined from generalized ellipsometry measurement of the near-band-gap order birefringence, are studied by far-infrared spectroscopic ellipsometry. We determine precise spectra of the anisotropic dielectric functions. With increasing $\ensuremath{\eta}$ all ${\mathrm{Ga}}_{0.52}{\mathrm{In}}_{0.48}\mathrm{P}$ phonon modes split in E- and ${A}_{1}$-type vibrations with polarization perpendicular and parallel to the sublattice ordering direction, respectively, and the mode frequencies shift continuously. We demonstrate here that the alloy-induced modes previously observed in highly disordered ${\mathrm{Ga}}_{0.52}{\mathrm{In}}_{0.48}\mathrm{P}$ [Phys. Rev. B 64, 155206 (2001)] have their origin in the ordered atomic arrangement of alike group-III atoms. We propose far-infrared dielectric anisotropy as a sensitive measure for sublattice ordering in multinary III-V alloys.

Stability, reconstruction, and surface electronic states of group-III atoms on SiC(111)

The adsorption of 1/3 monolayer of the group-III elements B, Al, Ga, and In on a Si-terminated $\mathrm{SiC}(111)\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}\mathrm{R}30\ifmmode^\circ\else\textdegree\fi{}$ surface is investigated in dependence on the surface preparation conditions using the density functional theory (DFT). The energetics, the bonding behavior and the resulting surface electronic structure are studied and chemical trends are derived. For the metal atoms Al, Ga, and In the replacement of a Si atom in a ${T}_{4}$ position always gives the most stable configuration. The first-row element B shows a completely different behavior. Under Si-rich conditions boron replaces a second-layer C atom in a ${S}_{5}$ position, whereas under C-rich conditions a first-layer Si atom is replaced by boron. The different adsorbate configurations result in completely different surface electronic structures. Al, Ga, and In adsorption passivates the surface. The fundamental gap is virtually free from empty or occupied surface states. The B adsorption in the ${S}_{5}$ configuration follows this behavior. However, boron substituting a Si-atom gives rise to an empty midgap surface band.

First-principles study of cation distribution in eighteen closed-shell AIIB2IIIO4 and AIVB2IIO4 spinel oxides

Using a first-principles band-structure method, we have systematically studied the cation distribution in closed-shell A{sup II}B{sub 2}{sup III}O{sub 4} and A{sup IV}B{sub 2}{sup II}O{sub 4} spinels where the group-II atoms are Mg, Zn, and Cd, the group-III atoms are Al, Ga, and In, and the group-IV atoms are Si, Ge, and Sn. The total energies, the structural parameters, and the band gaps of these compounds in both normal and inverse spinel structures are calculated. Compared with previous model studies, we show that an atomistic method is crucial to correctly identify the stability of the spinels and to calculate the anion displacement parameter u. The preference of cations with delocalized valence d states (e.g., Zn) to form covalent tetrahedral bonds also plays a significant role in determining the cation distribution in the spinels. Furthermore, the electronic structures of these spinel compounds depend strongly on the cation distribution. For most of the spinels studied here, the calculated band gaps for the inverse spinels are smaller than the corresponding normal spinels except for SnB{sub 2}{sup II}O{sub 4}.

Fluorine atoms in AlAs, GaAs, and InAs: Stable state, diffusion, and carrier passivation

We investigated various basic properties of fluorine (F) atoms in AlAs, GaAs, and InAs by using first-principles calculations. In these three semiconductors, we found that the $\ensuremath{-}1$ charge state is the most stable in a wide range of Fermi levels and that the F atom prefers sites surrounded by group-III atoms. These characteristics can be understood by considering that F has the largest electronegativity among the elements. We found that the diffusion properties, such as the diffusion path and the diffusion barrier height, are similar in these semiconductors. The estimated barrier heights are also comparable to the experimentally obtained barrier height in ${\mathrm{Al}}_{0.48}{\mathrm{In}}_{0.52}\mathrm{As},$ suggesting that the diffusion properties in ${\mathrm{Al}}_{x}{\mathrm{In}}_{1\ensuremath{-}x}\mathrm{As}$ are also similar. This implies that the experimentally observed selective F incorporation into ${\mathrm{Al}}_{x}{\mathrm{In}}_{1\ensuremath{-}x}\mathrm{As}$ is not due to the diffusion properties. It has been thought that the F atom forms a F-Si defect complex in ${\mathrm{Al}}_{x}{\mathrm{In}}_{1\ensuremath{-}x}\mathrm{As},$ but we found that a F atom in the $\ensuremath{-}1$ charge state has stability comparable to that of a F-Si defect complex in binary semiconductors, suggesting that F-Si defect complexes and isolated F atoms in the $\ensuremath{-}1$ charge state coexist in ${\mathrm{Al}}_{x}{\mathrm{In}}_{1\ensuremath{-}x}\mathrm{As}.$ This coexistence is consistent with the observation of the F-Si defect complex, as well as observed F doping effects such as donor passivation and the decrease in the electron mobility. Based on the present calculations for the binary semiconductors, some characteristic properties of F in ${\mathrm{Al}}_{x}{\mathrm{In}}_{1\ensuremath{-}x}\mathrm{As}$ can be understood.

III-V族化合物半導体ヘテロ界面におけるIII族原子分布のX線CTR散乱法による解析 : エピタキシャル成長III

III-V族化合物半導体ヘテロ界面におけるIII族原子分布のX線CTR散乱法による解析 : エピタキシャル成長III

X-ray CTR scattering measurement of InP/InGaAs/InP interface structures fabricated by different growth processes

Hetero-interfaces of InP/InGaAs (three monolayers (ML))/InP samples grown by organometallic vapor phase epitaxy (OMVPE) were investigated using X-ray crystal truncation rod (CTR) measurement. The samples were prepared with three different source-gas flow-sequences: (a) conventional sequence, (b) growth interruption, and (c) Ga and In source-gas pulse injection. The results of the X-ray CTR measurement showed that the interfaces between InP cap and InGaAs layers of (b) and (c) were sharper than that of (a) as intended. However, the amount of As in (b) was less than that designed since As atoms desorbed even from the InGaAs layer during the growth interruption. Surprisingly, the Ga atoms distributed wider and peak compositions of Ga were smaller than those designed for all the samples. It means that the quantum well structures are far from those designed and expected. It suggests that more efforts are necessary to control the distribution of group-III atoms after we solved the problem of group-V atom distributions.

Dynamic process of two-dimensional InAs growth in Stranski–Krastanov mode

In situ investigation of two-dimensional InAs layer, i.e., wetting layer, during the growth in the Stranski–Krastanov mode has been performed by using reflectance-difference (RD) spectroscopy and reflection high-energy electron diffraction (RHEED). RD spectra shows a dramatic change even at 0.1-monolayer (ML) deposition of InAs, which corresponds to a change of the surface reconstruction from the c(4×4) of the As-rich GaAs (001) to a (1×3) surface reconstruction. With advanced deposition, structures around 2.0 and 2.6 eV gradually change in the sign. The signal inversion at 2.6 eV indicates that the original As dimer along the [110] becomes dimerized along the [−110]. The 2.0 eV signal, which is related to electronic structures of group-III atoms and As bond, shows a different evolution during the growth. The intensity of the 2.0 eV signal shows the minimum around 1.0-ML deposition. After that, the intensity slightly increases until the start of the dot formation. This results indicates a decrease of In concentration in the wetting layer, i.e., a precursory transport process of In before the dot formation.

Structure of the metastable centers of the group-III atoms in IV-VI crystals

A comparative analysis of reconstruction of the metastable impurity centers in tetrahedral (III-V and II-VI) and cubic (IV-VI) crystals is performed for the case where the charge of the center changes. The influence of the chemical structure of the crystal matrix on the nature of defect reconstruction is studied on the basis of cluster approximation. For the IV-VI:III centers, the fundamental significance of the three-center crystal bonds and the specific role of the antibonding (weakly bonding) 3a1g state, which has the properties of both the impurity and perturbed crystal states simultaneously, are emphasized.

Diffusion in isotopically controlled semiconductor systems

Isotopically controlled heterostructures of 28Si/natSi and Al71GaAs/Al69GaAs/71GaAs have been used to study the self-diffusion process in this elemental and compound semiconductor material. The directly measured Si self-diffusion coefficient is compared with the self-interstitial and vacancy contribution to self-diffusion which were deduced from metal diffusion experiments. The remarkable agreement between the Si self-diffusion coefficients and the individual contributions to self-diffusion shows that both self-interstitials and vacancies mediate Si self-diffusion. The Ga self-diffusion in undoped AlGaAs was found to decrease with increasing Al concentration. The activation enthalpy of Ga and Al diffusion in GaAs and of Ga diffusion in AlGaAs all lie in the range of (3.6±0.1)eV, but with different pre-exponential factors. The doping dependence of Ga self-diffusion reveals a retardation (enhancement) of Ga diffusion under p-type (n-type) doping compared to intrinsic conditions. All experimental results on the group-III atom diffusion are accurately described if vacancies on the group-III sublattice are assumed to mediate the Ga self- and Al–Ga interdiffusion in undoped AlGaAs and the Ga self-diffusion in Be- and Si-doped GaAs with an active dopant concentration of 3×1018cm−3. The doping dependence of Ga self-diffusion in GaAs provides strong evidence that neutral, singly and doubly charged Ga vacancies govern the self-diffusion process.

Enhanced and retarded Ga self-diffusion in Si and Be doped GaAs isotope heterostructures

Ga self-diffusion in undoped, Si- and Be-doped 71GaAs/ n at GaAs isotope heterostructures has been investigated at temperatures between 736°C and 1050°C. Concentration profiles of the Ga isotopes measured separately by secondary ion mass spectrometry reveal retarded (enhanced) self-diffusion when doped p-type (n-type), compared to intrinsic conditions. Electrochemical C–V profiling was performed to determine the free carrier concentrations before and after annealing. Detailed analysis of the doping dependence of Ga self-diffusion includes a compensation of Si donors by negatively charged vacancies and shows that one native defect, presumably the vacancy on the Ga sublattice, in the neutral, singly and doubly negatively charged states governs the self-diffusion process. The energy levels E V − − E v =0.42 eV and E V 2− − E v =0.60 eV of the singly and doubly negatively charged gallium vacancy have been deduced from the doping dependence of Ga self-diffusion. No significant contribution from the triply negatively charged Ga vacancy has been found. Our evaluation of the doping dependence of Ga self-diffusion is compared with literature data for the group-III atom diffusion in the AlGaAs material system. Seemingly inconsistent literature data can now be described consistently on the basis of our results. Finally, thermal equilibrium concentrations and transport coefficients of Ga vacancies in GaAs in their various charge states are calculated for different doping conditions taking into account the vacancy related energy levels and the thermal equilibrium concentration of neutral vacancies which were deduced from our self-diffusion data.