The effect of spacer-layer growth temperature on mobility in a two-dimensional electron gas in PHEMT structures

The effect of growth temperature of the AlGaAs spacer layer on mobility in a two-dimensional electron gas μ _e in single-side δ-doped pseudomorphic AlGaAs/InGaAs/GaAs transistor structures with a high electron mobility is studied experimentally. The energy-band diagram is analyzed using a self-consistent calculation. In order to study the electronic transport properties, an optimized structure in which there is no parallel conduction over the doped layer was chosen. It is shown that, in optimized structures, the mobility μ _e increases by 53% at T = 300 K and by 69% at T = 77 K as the growth temperature increases from 590 to 610°C, with the other parameters and the growth conditions remaining the same. It is assumed that this behavior is related to an improvement in the structural quality of the AlGaAs spacer layer and the AlGaAs/InGaAs/GaAs heteroboundary.     

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.     

Long-wavelength light-emitting diodes (λ=3.4–3.9 µm) based on InAsSb/InAs heterostructures grown by vapor-phase epitaxy

InAs/InAs_0.93Sb_0.07/InAs heterostructures were grown by metal-organic vapor-phase epitaxy in a horizontal reactor at atmospheric pressure. Based on the obtained structures, light-emitting diodes operating at λ=3.45 μm (T=77 K) and λ=3.95 μm (T=300 K) were fabricated. The room-temperature quantum efficiency of light-emitting diodes was 0.12%. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 34, No. 12, 2000, pp. 1462–1467. Original Russian Text Copyright ? 2000 by Zotova, Kizhaev, Molchanov, Popova, Yakovlev.     

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.     

Hot electrons and phonons under high intensity photoexcitation of semiconductors

It has become well established during the last few years that intense photoexcitation of a semiconductor leads to the heating of the carriers and the generation of nonequilibrium phonons. These phenomena which result from the relaxation of photoexcited carriers to the band extrema by interaction with other carriers and by emission of phonons, are reviewed in this paper. At relatively low intensities (<10⁵W/cm² for GaAs) the photoexcited carrier distribution is Maxwellian with a carrier temperature T_e different from the lattice temperature. T_e as high as 150K and effective phonon temperatures as high as 3700K have been observed in GaAs. The observed variation of T_e with excitation intensity leads to the conclusion that in semiconductors like GaAs the polar optical mode scattering is the dominant energy loss mechanism from the electron gas to the lattice. Similar results are obtained in CdSe and CdS. At higher intensities (>10⁵W/cm² for GaAs), the carrier dist0ribution becomes non-Maxwellian for reasons not well understood at present. We will also discuss some recent measurements of variation of T_e with excitation wavelength and of the transmission spectra of photoexcited GaAs.     

Magnetic ordering effects in heavily doped GaAs:Fe crystals

The exchange coupling of Fe centers in GaAs crystals is studied by electron spin resonance (ESR). Transitions to a superparamagnetic state and to an impurity ferromagnetism domain are analyzed. A study of a system of single-domain magnetically ordered regions in GaAs:Fe with the transition to a ferromagnetic state occurring at the temperature T_C1 = 460 K is described. It is shown that impurity ferromagnetism with a transition temperature T_C2 of 60 K in a disordered system of Fe centers randomly distributed among superparamagnetic regions exists in GaAs:Fe.     

High-field properties of n-InP under high pressure

The high electric field properties of n-InP at 300 K have been studied as a function of pressure. Hydrostatic measurements are made in a piston and cylinder apparatus, using a liquid pressure-transmitting medium. The threshold fields (E_T) for transferred electron instabilities range from 7.5 to 8.5 kV/cm at atmospheric pressure. The resistivity of the samples increases with increasing pressure. The most reliable results show that E_T increases slightly with pressure below 40 kbar. This behavior can be explained qualitatively in terms of possible band structure changes. By using known variations of parameters such as effective mass and sub-band energy gaps, detailed theoretical calculations are carried out to fit the data and to determine the correct mode of operation (two- or three-level operation). The results are also compared with analogous experiments on GaAs.     

Analysis of inelastic scattering of quasi-two-dimensional electrons of a superlattice by acoustic phonons with allowance made for the miniband dispersion

Formulas for the effective momentum-relaxation time and mobility of quasi-two-dimensional electrons of a superlattice with consideration of inelastic scattering by acoustic phonons and the dispersion of the miniband energy spectrum as a function of the longitudinal wave vector have been obtained. Numerical calculation was performed for a nondegenerate gas of quasi-2D electrons in a symmetrical GaAs/Al_0.36Ga_0.64As superlattice with a quantum well width of 5 nm at T=77 K. It was shown that consideration of the elasticity of scattering and the dispersion of the miniband energy spectrum gives rise to a significant increase in the electron mobility.     

Optical transitions of Al0.35Ga0.65As/GaAs asymmetric double quantum wells grown on GaAs(n11)A (n≤4) substrates

The optical transitions in Al_0.35Ga_0.65As/GaAs asymmetric double quantum wells (ADQWs) grown on GaAs(n11)A (n≤4) substrates were studied by photoluminescence at low temperatures. The redshift of two PL peaks for substrate orientation far from the (100) plane is attributed to the large anisotropy of the heavy-hole band in the different GaAs orientations. The energy difference of the two transitions also shows a similar shift. By comparing the PL measurements with a simple effective mass calculation, the effective mass of the heavy hole on GaAs(n11)A (n=1, 2, 3, 4) planes is found to be about 0.95, 0.73, 0.54 and 0.41m₀, respectively. The effect of As pressures on PL peak energies in ADQWs is also studied. The As pressure-dependence of PL peak energies is attibuted to a decrease of the Ga desporption rate with increasing As pressures on GaAs(n11)A substrates.     

Investigation of the physical properties of semiconductor nanostructures using samples with laterally nonuniform layers: 1. Photoluminescence

A new technique for the experimental investigation of semiconductor structures is suggested and implemented. The technique is based on an analysis of correlations in the spectra of samples with laterally nonuniform layers. A molecular-beam-epitaxy-grown sample containing Al_xGa_1?xAs-GaAs and GaAs-In_yGa_1?yAs quantum wells (QWs) and a modulation doped Al_xGa_1?xAs-GaAs heterojunction was studied by photoluminescence (PL) spectroscopy at 77 K. The dependences of the PL spectra on the parameters describing sample nonuniformity were analyzed, which made it possible to characterize the processes of the charge-carrier redistribution in the structure and to reveal a number of specific features in the PL of narrow GaAs QWs. In the entire range of the nonuniformity-related variation in the semiconductor structure parameters, the values of the optical transition energies determined experimentally agree with those calculated theoretically and may serve as a basis for estimating these parameters. It is shown in this study that the suggested approach is highly informative, which stems from the capacity for precision control over the technologically adjustable parameters of the structure within the same sample.     

High Tc superconducting three-pole parallel-coupled bandpass filters made from laser ablated YBa2Cu3O7 − δ thin films

We have investigated three-pole parallel-coupled bandpass filters with fractional bandwidths of about 6% and 3% at a center frequency of 10.5 GHz utilizing high T_c superconducting YBa₂Cu₃O_{7 − δ} thin films. The films were deposited on LaAlO₃ substrates by pulsed laser ablation. Microwave responses of the filters were measured as a function of temperature and input power. The performance of the 3% bandwidth filter exhibits low insertion losses of about 0.48 dB and 0.79 dB at 20 K and 77 K, respectively. The insertion loss in the 6% bandwidth filter was 0.96 dB at 77 K. Both filters showed return losses better than 15 dB. High T_c superconducting bandpass filters showed good reproducibility. They were also compared with equivalent gold filters which showed insertion losses of more than 8 dB at 77 K.     

Characterization of GaxIn1−xAs grown with TMIn

High quality Ga_xIn_1−xAs, lattice matched to InP, has been reproducibly grown by organometallic vapor phase epitaxy using trimethylgallium (TMGa), trimethylindium (TMIn), and AsH₃ in an atmospheric pressure reactor with no observable adduct formation. For the first time, using TMIn, room temperature electron mobilities of 10⁴ cm²/Vs and 77 K mobilities greater than 4 × 10⁴ cm²/Vs have beep obtained. Residual donor doping densities in the low 10¹⁵ cm⁻³ range have been routinely obtained. Material with excellent morphology has been grown from 540 to 670 C with the highest quality material being obtained near 650 C. The 4 K photoluminescence (PL) peak due to carbon is not seen in the material grown at higher temperatures; however, it increases dramatically as the growth temperature is lowered. This increased carbon incorporation leads to a sharp drop in the electron mobility, which exhibits a T^−0.5 behavior between 77 and 300 K. With optimum growth conditions, 4 K PL halfwidths of 4–5 meV are commonly observed. This high quality material is characterized by x-ray diffraction, PL, and Hall mobility measurements. Carbon and other impurity incorporation as a function of the growth parameters will be described.     

Infrared studies of be-doped GaAs grown by molecular beam epitaxy at low temperatures

Samples of molecular beam epitaxial GaAs grown at low temperatures doped with Be defects are studied as a function of growth temperature (T_G)-by measuring their localized vibrational modes at 77K using BOMEM Fourier transform infrared spectrometer. Localized vibrational modes of⁹Be_Ga in samples grown at T_G>350°C have been identified at 482 cm⁻¹. Secondary ion mass spectroscopy measurements show that the densities of Be defects remain approximately constant as T_G is lowered, however, additional structure in the⁹Be_Ga localized vibrational mode is observed. Calculations based on Green's function theory suggest that the additional structure in Be-doped LT GaAs can best be explained in terms of a complex center [⁹Be_Ga-As_Ga] involving an intrinsic defect.     

Radiative recombination in Ge<Superscript>+</Superscript>-implanted SiO<Subscript>2</Subscript> films annealed under hydrostatic pressure

Photoluminescence (PL) spectra and PL excitation spectra were recorded at room temperature from SiO₂ films implanted with Ge⁺ ions and annealed at temperature T _a =450–1100°C under hydrostatic pressure P=12 kbar. The emergence of features in the violet and green bands of the PL and PL excitation spectra correlates with the formation of hydrostatically strained Ge nanocrystals. The shift of the PL bands to higher energies, which occurs as the annealing temperature is raised to T _a ≥800°C, can be attributed to a shift of the energy levels related to the radiative recombination centers, which is caused by the increasing deformation potential. The observed PL is accounted for by the enhanced probability of direct radiative transitions in Ge nanocrystals with an X-like conduction band.     

Inversion of the electron population in subbands of dimensional quantization with longitudinal transport in tunnel-coupled quantum wells

The scheme of a laser which can operate in the far-infrared range (λ ∼ 150 μm) is suggested. In order to attain the inversion of the subband population it was suggested that electron transport in three tunnel-coupled quantum wells in a strong electric field, which lies in the plane of quantum wells, be used. An important specific feature of the structure suggested is the presence of a single rough heterointerface. The electron trans-port was simulated by the Monte Carlo method for the Al_xGa_1−x As/GaAs (x=0.2–0.3) heterostructure. The simulation demonstrated that the population inversion in the first and second subbands of dimensional quantization is realized in the field above 1.2 kV/cm at T=4.2 and 77 K. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 36, No. 6, 2002, pp. 724–729. Original Russian Text Copyright ? 2002 by Aleshkin, Dubinov.     

Elimination of a DX-center by an AlAs/n-GaAs superlattice and its application to 2DEGFETs

AlAs/n-GaAs superlattices (super-doped structure), in which Si donors were doped only in GaAs mid-layers, were examined comparing them with n-type Al_xGa_1−xAs (x>0.25) alloys and various selectively doped short-period superlattices (<40 Å). It has been confirmed that only the super-doped structure among the superlattices realizes the elimination of DX-centers and makes shallow levels. Using the super-doped structure as electron-supplying layers, normal and inverted two-dimensional electron gas FETs (2DEGFETs) were successfully fabricated. The threshold voltage shifts in temperature (77–300 K) were effectively suppressed to 0.10 V (V_T=−0.38∼−0.48 V) and 0.18 V (V_T=−1.48∼−1.66 V) for normal and inverted 2DEGFETs.     

Optical and transport properties of δ-doped pseudomorphic AlGaAs/InGaAs/GaAs structures

We investigate the effects of spacer layer thickness on the optical and transport properties of the n-typeδ-doped pseudomorphic Al_0.30Ga_0.70As/In_0.15Ga_0.85As / GaAs structures. Aδ-doped AlGaAs/InGaAs/GaAs structure with a 6nm spacer layer yields a sheet carrier concentration of 1.5×10¹² cm^?2 at 77K with electron mobility of 6.4×10³ cm²/Vs, 3.11×10⁴ cm²/Vs, and 3.45×10⁴ cm²/Vs at room temperature, 77 and 20K, respectively. The effects of the different scattering mechanisms on luminescence linewidth and electron mobility have also been discussed.     

Study of the correlation between structural and photoluminescence properties of CdSe thin films deposited by close-spaced vacuum sublimation

CdSe polycrystalline films were deposited by a close-spaced vacuum sublimation method at different substrate temperatures (T_s) using glass slides as substrates. At T_s≤673 K the films have a structure with strong dispersion of grain size (d) (from 0.1 to 0.3 μm). In this case the layer-by-layer mechanism determines the growth process of the layers. For T_s=873 K they have a columnar-like structure with a clear growth texture and the average grain size d=3–4 μm. The films obtained at T_s>473 K are n-type and only correspond to a single wurtzite phase. The crystallites are preferentially oriented with the (102) planes parallel to the substrate. At lower temperatures the films are bi-phase. The microstress level in CdSe films obtained at Т_s=873 K (0.5×10⁻³) is considerably smaller than for the films deposited at Т_s=773 K (4.0×10⁻³). Increase of the value of T_s improves the stoichiometry of CdSe films. Analysis of the low-temperature photoluminescence (PL) spectra let us determine the nature and energy of point and extended defects in the investigated films. It was shown that the films contain Na(Li) and P residual impurities. The results of the structural and PL measurements showed that the CdSe polycrystalline films are of fairly good crystal and optical quality for T_s=873 K and can be suitable for various applications.     

Photoluminescence study of AlGaAs/GaAs/AlGaAs double quantum wells separated by a thin AlAs layer

Photoluminescence (PL) spectra of Al_0.21Ga_0.79As/GaAs/Al_0.21Ga_0.79As double quantum wells (DQWs) separated by a thin AlAs barrier have been studied in the temperature range 77–300 K. The well width was varied from 65 to 175 Å, and the thickness of the AlAs barrier was 5, 10, or 20 Å. In the case of a sufficiently thin (5, 10 Å) AlAs barrier, the energy spectrum of QW states is considerably modified by coupling between the QWs. This effect shifts the main spectral peak of PL, and specific features associated with the splitting of the ground state into symmetric and asymmetric states are observed in the spectra at higher temperatures. The DQW structure with a 20-Å-thick AlAs barrier is a system of two uncoupled asymmetric Al_0.21Ga_0.79As/GaAs/AlAs QWs. The energy levels in double coupled QWs were calculated as functions of the well width and AlAs barrier thickness, and good correlation with the experimentally observed energies of optical transitions was obtained.     

Dispersion of the relaxation time of quasi-two-dimensional electrons under conditions of ionized-impurity scattering in a superlattice with doped quantum wells

Ionized-impurity scattering of quasi-two-dimensional electrons in GaAs/Al_0.36Ga_0.64As superlattices (SLs) with doped quantum wells is considered. The scattering probability is calculated assuming that the electron wave function is represented by the ground-state eigenfunction of the lowest SL miniband. Analytical expressions describing dispersion of the longitudinal and transverse relaxation times with respect to the longitudinal wave vector are derived and numeric calculations are carried out. The dependence of the components of the relaxation-time tensor on the SL period and the temperature in the region around T=77 K is analyzed.