Annealing of AsGa-related defects in LT-GaAs: The role of gallium vacancies

We have studied the annealing properties of As_Ga-related defects in layers of GaAs grown at low substrate temperatures (300°C) by molecular beam epitaxy (low temperature[LTx]-GaAs). The concentration of neutral As_Ga-related native defects, estimated by infrared absorption measurements, ranges from 2×10¹⁹ to 1×10²⁰ cm⁻³. Slow positron annihilation results indicate an excess concentration of Ga vacancies in LT layers over bulk grown crystals. A sharp annealing stage at 450°C marks a rapid decrease in the As_Ga defect concentration. We propose that the defect removal mechanism is the diffusion of As_Ga to arsenic precipitates, which is enhanced by the presence of excess V_Ga. The supersaturated concentration of V_Ga must also decrease. Hence, the diffusivity of the As_Ga defects is time dependent. Analysis of isothermal annealing kinetics gives an enthalpy of migration of 2.0±0.3 eV for the photoquenchable As_Ga defects, 1.5±0.3 eV for the V_Ga, and 1.1±0.3 eV for the nonphotoquenchable defects. The difference in activation enthalpy represents difference energy between an As atom and Ga atom swapping sites with a V_Ga.     

Photoluminescence of CuGaTeAs and CuGaSnGa complexes in n-GaAs under resonance polarized excitation

Photoluminescence (PL) of n-type GaAs:Te:Cu and GaAs:Sn:Cu with an electron density of about 10¹⁸ cm^?3 was studied at 77 K. A broad band with a peak at the photon energy near 1.30 eV (GaAs:Te:Cu) or 1.27 eV (GaAs:Sn:Cu) was dominant in the PL spectrum under interband excitation. This band arose from the recombination of electrons with holes trapped by Cu_GaTe_As or Cu_GaSn_Ga complexes. It has been found that the low-energy edge of the excitation spectrum of this PL band at photon energies below ～1.4 eV is controlled by the optical ejection of electrons from a complex into the conduction band or to a shallow excited state. The PL polarization factors upon excitation by polarized light from this spectral range suggest that the complexes have no additional distortions caused by an interaction of a hole bound at the center in the light-emitting state with local phonons of low symmetry. This feature makes Cu_GaTe_As and Cu_GaSn_Ga complexes different from those with the Ga vacancy (V _Ga) instead of Cu_Ga. The dissimilarity arises from the difference in the intensity of interaction of a hole localized at the orbital of an isolated deep-level acceptor in the state corresponding to its preemission state in the complex (Cu _Ga ^? and V _Ga ^2? ) with low-symmetry vibrations of atoms. The perturbation of the hole orbital induced by the donor in the complex practically does not affect this interaction.     

Comparison of the polarizations of the 1.2-eV photoluminescence band in n-GaAs:Te under uniaxial pressure and resonance polarized excitation

It is shown that the photoluminescence (PL) band at 1.2 eV in n-GaAs:Te, which is associated with emission from V _GaTe_As complexes with reorienting Jahn-Teller distortions, also includes a contribution from nonreorienting defects. The optical dipole parameters are almost the same for both types of defects. Expressions relating the polarization of the PL band at 1.2 eV under uniaxial pressure and polarized resonant excitation to dipole parameters and to relative contributions to emission from reorienting and nonreorienting defects are derived. A procedure is developed for evaluating these characteristics by analyzing experimental data, and the contributions from each kind of defects to the PL band at 1.2 eV were found to be comparable, even though they vary from sample to sample. The obtained angles characterizing the position of the axes of optical dipoles associated with the defects in light-absorbing and light-emitting states indicate that, in the former state, the effects of donors and the Jahn-Teller distortion on the vacancy orbitals of the V _GaTe_As complex are comparable, while in the latter, the effect of distortion is dominant.     

Antisite arsenic incorporation in the low temperature MBE of gallium arsenide: Physics and modeling

A stochastic model for simulating the surface growth processes in the low temperature molecular beam epitaxy of gallium arsenide is developed to investigate the incorporation of antisite As and its dependence on the growth conditions including the dynamics of the physisorbed As on the surface. Three different kinetic models with a combination of surface kinetic processes such as incorporation of antisite As, evaporation of antisite As and incorporation of regular As. The kinetic model with all three surface processes was accepted as the best model due to its physical soundness and reasonableness of its model parameters. The arsenic flux, temperature, and growth rate dependences of antisite arsenic (As_Ga) obtained from our simulation are in excellent agreement with the experimental results. The activation energy of 1.16 eV and a frequency factor of 4×10¹²/s for the evaporation of antisite arsenic obtained from our model are in good agreement with experimental and theoretical estimates. At a constant substrate temperature and growth rate, the antisite arsenic concentration increases with arsenic flux for low fluxes and saturates beyond a critical flux. The critical arsenic flux increases with temperature and the saturation value of the As_Ga concentration decreases with temperature. As the arsenic flux increases, the coverage of the physisorbed layer increases and at a critical flux dictated by the fixed temperature and growth rate, the coverage saturates at its maximum value of unity (a complete monolayer) and hence the concentration of As_Ga saturates. Lower As_Ga concentration results at higher temperature due to more evaporation of As_Ga. Additionally, an analytical model is developed to predict the As_Ga concentration for various growth conditions.     

Molecular beam epitaxial growth of MgZnCdSe on (100) InP substrates

Wide-gap II-VI MgZnCdSe quaternary compounds were grown on InP substrates by molecular beam epitaxy, for the first time. Changing the Mg composition (x = 0 to 0.63), various Mg_x(Zn_yCd_{1_y})_{1_x}Se lattice-matched to InP were grown. Mirror-like surface morphologies and streaky reflection high energy electron diffraction patterns of MgZnCdSe were obtained. With increased Mg compositions, the band-edge emissions wavelength in photoluminescence spectra was shifted from 572 nm (2.17 eV) to 398 nm (3.12 eV) at 15K. Furthermore, the absolute PL peak intensity increased drastically with increased band-edge emission, being accompanied by a relative decrement in the deep level emission intensities were also observed.     

Deep level investigation on n-In0.35Ga0.65As/GaAs structures

We have investigated the deep electronic levels in n-In_0.35Ga_0.65As epitaxial layers grown by molecular beam epitaxy (MBE) on graded In_xGa_1−xAs buffer/GaAs structures. In_xGa_1−xAs buffer layers with linear, parabolic and power composition grading law, respectively have been considered. The dependence of the deep levels distribution on the buffer grading law as well as on growth parameters such as the growth temperature and use of As₂ or As₄ beams is reported.     

Photoluminescence of V-doped GaN thin films grown by MOVPE technique

This work reports the photoluminescence (PL) study of vanadium-doped GaN (GaN: V) in the 9-300 K range. Samples have been successfully prepared on sapphire substrates by metalorganic vapour phase epitaxy technique (MOVPE). At room temperature (RT) the PL spectra of GaN: V are dominated by a blue band (BB) in the 2.6 eV range. This BB emission is very strong and its intensity increases with increasing V doping level. We also observed that the peak position of the blue luminescence shifted at lower energy with decreasing excitation density. Upon V-doping, the yellow luminescence band shows a drastic reduction in integrated intensity. This observation is explained by a reaction involving V and gallium vacancy (_√Ga). PL spectra at low temperature exhibited a series of peaks. The donor-acceptor (D-A) pair emission peak at 3.27 eV was strongly pronounced, as the temperature was decreased. On the other hand, the intensity of the BB emission decreased. This BB emission is due to a radiative transition from a shallow donor with a depth of 29 meV to a deep acceptor with a depth of 832 meV.     

The effects of the growth temperature on AlxGal-xAs (0≤ x ≤0.37) LED materials grown by OM-VPE

The epitaxial growth of AlGaAs of LED quality by OM-VPE is achieved either by using high growth temperatures (≥780°C) or by using oxygen gettering methods and low growth temperatures (≤750°C). For 6% AlGaAs, the most studied composition in this work, graphite baffles and a molecular sieve, are used at low growth temperatures (≈680°C) to improve both the normalized PL intensity of epi layers and the EL efficiency of LEDs. Growth at high temperature, however, does not require oxygen gettering methods to achieve the same material quality. The improvement in both cases is achieved by reducing the concentration of the oxygen-related defect that is the source of the 0.8 eV peak, which limits the performance of 6% AlGaAs LEDs. Al_xGa_1−xAs alloys with x≥0.06 also show a similar behavior relative to the growth temperature. The defect peak itself remains invariant with x. This 0.8 eV PL peak is likely to be associated with Al, since the reaction between Al and oxygen is strong and the 0.8 eV peak is seldom observed in GaAs epi layers, regardless of the growth temperature.     

OMVPE growth of GaInAsSb/AlGaAsSb for quantum-well diode lasers

GaInAsSb and AlGaAsSb alloys have been grown by organometallic vapor phase epitaxy (OMVPE) using all organometallic sources, which include tritertiarybutylaluminum, triethylgallium, trimethylindium, tertiarybutylarsine (TBAs), and trimethylantimony. Excellent control of lattice-matching both alloys to GaSb substrates is achieved with TBAs. GaInAsSb/AlGaAsSb multiple quantum well (MQW) structures grown by OMVPE exhibit strong 4K photoluminescence with full width at half maximum of 10 meV, which is comparable to values reported for quantum well (QW) structures grown by molecular beam epitaxy. Furthermore, we have grown GaInAsSb/AlGaAsSb MQW diode lasers which consist of n- and p-doped Al_0.59Ga_0.41As_0.05Sb_0.95 cladding layers, Al_0.28Ga_0.72As_0.02Sb_0.98 confining layers, and four 15 nm thick Ga_0.87In_0.13As_0.12Sb_0.88 quantum wells with 20 nm thick Al_0.28Ga_0.72As_0.02Sb_0.98 barrier layers. These lasers, emitting at 2.1 μm, have exhibited room-temperature pulsed threshold current densities as low as 1.2 kA/cm².     

Luminescent properties of BexCd1−xSe thin films

We report photoluminescence (PL) study of Be_xCd_1−xSe epitaxial layers (x<0.21) grown by molecular beam epitaxy on InP substrates. Continuous wave PL spectra are taken within a 4.2-300 K temperature range. We observe an anomalous ‘s-shaped’ temperature dependence of emission energy and a severe decrease of emission intensity with the increase of temperature. We explain an ‘s-shaped’ temperature dependence of emission energy by exciton localization in the potential minima at low temperatures followed by thermal activation at higher temperatures. We attribute low emission intensity at high temperatures to exciton dissociation and electron/hole migration to non-radiative recombination centers.     

Photoluminescence spectra of Si1-xGex/Si quantum well structures grown by three different techniques

Photoluminescence (PL) spectra of Si_1-xGe_x/Si multiple quantum wells have been measured at 4.2 K for the samples grown by three different techniques; conventional molecular beam epitaxy (MBE), gas-source MBE, and ultra high vacuum chemical vapor deposition (UHV-CVD). Only in the case of conventional MBE, strong emission bands appear about 80 meV below the band gap of Si_1-xGe_x. These strong emission bands disappear after the annealing at 800° C. From the dependence of the PL intensity on the excitation power, strong emission is considered to be due to some recombination center. On the other hand, in the case of gas-source MBE and UHV-CVD, the strong emission bands are undetectable, although the band-edge PL lines of Si_1-xGe_x are clearly observed. There is no significant change in the PL spectra after the annealing. The origin of the strong emission band is considered to be defects which are characteristic of conventional MBE.     

Characteristics of deep levels in Al-doped ZnSe grown by molecular beam epitaxy

We investigated the characteristics of deep levels in heavily Al-doped ZnSe layers grown by molecular beam epitaxy, whose electron concentration is saturated. Low-temperature photoluminescence showed deep level emission around 2.25 eV, and its intensity increases with Al concentration. This deep-level is located at 0.55 eV above valence band maximum, implying a point defect such as a self-activated center, Al_ZnV_Zn. Deep-level transient spectroscopy was used to investigate non-radiative trap centers in Al-doped ZnSe layers, and showed the presence of two electron trap centers at depths of 0.16 and 0.80 eV below conduction band minimum, with the electron capture cross-sections of 810⁻¹² and 1×10⁻⁷ cm², respectively. It is suggested the carrier compensation in heavily Al-doped ZnSe layers be ascribed to the deep levels.     

Group-V composition control for InGaAsP grown by gas source molecular beam epitaxy

Based on our kinetics models for gas source molecular beam epitaxy of mixed group-V ternary materials, the group-V composition control in In_yGa_1−yAs_1−xP_x epilayers has been studied. The P or As composition in In_yGa_1−yAs_1−xP_x (lattice matched to InP or GaAs) can be obtained from a simple equation for substrate temperatures below 500°C. This has been verified by a series of experimental results.     

Stoichiometry-dependent deep levels in undoped p-type Al0.38Ga0.62As grown by liquid phase epitaxy

Photocapacitance (PHCAP) and photoluminescence (PL) measurements were applied to unintentionally doped p-type Al_0.38Ga_0.62As grown by liquid phase epitaxy using the temperature difference method under controlled vapor pressure. PHCAP spectra revealed three dominant deep levels at E_v+0.9, E_v + 1.45, and E_v+1.96 eV, and a deep level at E_v+0.9−1.5 eV which was not neutralized by forward bias injection. These level densities increase with increasing arsenic vapor pressure and net shallow acceptor density. Furthermore, PL spectra reveal a deep level at 1.6–1.7 eV. The PL intensity of this deep level increases with increasing arsenic vapor pressure. These deep levels are thought to be associated with excess As.     

Femtosecond optical response of low temperature grown In0.53Ga0.47As

A femtosecond, tunable color center laser was used to conduct degenerate pump-probe transmission spectroscopy of thin film low temperature grown molecular beam epitaxy In_0.53Ga_0.47As samples. Low temperature molecular beam epitaxy In_0.53Ga_0.47As exhibits a growth-temperature dependent femtosecond optical response when probed near the conduction band edge. Below T_g=250°C, the optical response time of the material is subpicosecond in duration, and we observed induced absorption, which we suggest is due to the formation of a quasi-“three-level system”.     

A coherent model for deep-level photoluminescence of cu-contaminated n-type gaas single crystals

A comparative study of the 77 K PL spectra of about one hundred n-type GaAs single crys-tals, grown by the horizontal or Czochralski technique, shows that the principal native defects in the former (in the “as-grown” state) are As-vacancies, whereas Ga-vacancies are dominating the latter (2). Besides the VGa, copper appears as the major acceptor contaminant in these crystals and is respon-sible for the 0.83 eV peak. Association of Cu and/or V_Ga with V_As and/or dopant atoms tends to neutralise this compensation. Considering the contradicting attributions found in the literature, new definitions of the nature of some centers responsible for the deep-level PL were needed to fit with the experimental da-ta. Thus the center yielding the 1.35 eV, PL-band, usually identified as substitutional copper, has to be defined here as the As-va-cancy associated defect complex: (V_As+Cu=V_As+) Similarly the 1.295 eV PL is associated with the (Te+Cu=V_As+ complex and the often en-countered level at 1.22 eV is identified as the (Si+Cu=V_As ⁺) complex. These levels, to-gether with the better known ones at 0.83, 0.96, 1.02, 1.17, 1.19 eV are linked in a coherent model which can account for the ap-pearance and the relative amplitude of the different PL peaks observed, as a result of the doping and the thermal history of the crystals during growth.     

Characterization of AlxInyGa1−x−yN quaternary alloys grown on sapphire substrates by molecular-beam epitaxy

High-resolution X-ray diffraction (HR-XRD) with rocking curve, atomic force microscopy (AFM) and photoluminescence (PL) spectroscopy have been performed on high-quality quaternary Al_xIn_yGa_1−x−yN thin films at room temperature. The Al_xIn_yGa_1−x−yN films were grown on c-plane (0 0 0 1) sapphire substrates with AlN as buffer layers using a molecular beam epitaxy (MBE) technique with aluminum (Al) mole fractions x ranging from 0.0 to 0.2 and constant indium (In) mole fraction y=0.1. HR-XRD measurements confirmed the high crystalline quality of these alloys without any phase separation. The X-ray rocking curve of Al_xIn_yGa_1−x−yN films typically shows full widths at half maximum (FWHM) intensity between 14.4 and 28.8 arcmin. AFM measurements revealed a two-dimensional (2D) growth mode with a smooth surface morphology of quaternary epilayers. PL spectra exhibited both an enhancement of the integrated intensity and an increasing blueshift with increased Al content with reference to the ternary sample In_0.1Ga_0.90N. Both effects arise from Al-enhanced exciton localization. PL was used to determine the behavior of the energy band gap of the quaternary films, which was found to increase with increasing Al composition from 0.05 to 0.2. This trend is expected since the incorporation of Al increases the energy band gap of ternary In_0.1Ga_0.90N (3.004 eV). We have also investigated the bowing parameter for the variation of energy band gaps and found it to be very sensitive on the Al composition. A value of b=10.4 has been obtained for our quaternary Al_xIn_yGa_1−x−yN alloys.     

Photoluminescence study on coarsening of self-assembled InAlAs quantum dots on GaAs (001)

Red-emission at ∼640 nm from self-assembled In_0.55Al_0.45As/Al_0.5Ga_0.5As quantum dots grown on GaAs substrate by molecular beam epitaxy (MBE) has been demonstrated. We obtained a double-peak structure of photoluminescence (PL) spectra from quantum dots. An atomic force micrograph (AFM) image for uncapped sample also shows a bimodal distribution of dot sizes. From the temperature and excitation intensity dependence of PL spectra, we found that the double-peak structure of PL spectra from quantum dots was strongly correlated to the two predominant quantum dot families. Taking into account quantum-size effect on the peak energy, we propose that the high (low) energy peak results from a smaller (larger) dot family, and this result is identical with the statistical distribution of dot lateral size from the AFM image.     

Optical study of InP quantum dots

Results of photoluminescence (PL) studies of self-organized nanoscale InP islands (quantum dots, QDs) in the In_0.49Ga_0.51P matrix, grown on a GaAs substrate by metalorganic vapor phase epitaxy (MOVPE), are presented. Dependences of the PL efficiency on temperature in the range 77–300 K and on excitation level at pumping power densities of 0.01–5 kW/cm² have been obtained. The PL spectra are a superposition of emission peaks from QDs and the wetting layer. Their intensity ratio depends on the pumping power and temperature, and the emission wavelength varies in the range 0.65–0.73 μm. At 77 K and low excitation level, InP QDs exhibit high temperature stability of the emission wavelength and high quantum efficiency. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 35, No. 2, 2001, pp. 242–244. Original Russian Text Copyright ? 2001 by Vinokurov, Kapitonov, Nikolaev, Sokolova, Tarasov.     

Room-temperature photoluminescence of Ga<Subscript>0.96</Subscript>In<Subscript>0.04</Subscript>As<Subscript>0.11</Subscript>Sb<Subscript>0.89</Subscript> lattice matched to InAs

Photoluminescence (PL) has been observed at room temperature from a Ga_0.96In_0.04As_0.11Sb_0.89 quaternary solid solution for the first time. High-quality epitaxial layers of n-type (Te-doped) Ga_0.96In_0.04As_0.11Sb_0.89 with low In content were grown by liquid phase epitaxy (LPE) lattice-matched to InAs(100) substrates from a Ga-rich melt. The PL properties of the material were investigated over a wide temperature range, and the principal radiative transitions were identified. In the temperature range <150 K, donor-acceptor recombination involving the first and second ionization state of native antisite defects was the dominant radiative-recombination process, whereas interband recombination was found to dominate at room temperature.