Acoustically driven charge separation in semiconductor heterostructures sensed by optical spectroscopy techniques

We demonstrate a method of using a two-layer sandwich structure, which includes a LiNbO₃ plate and a semiconductor heterostructure to create an inhomogeneous stress and piezoelectric harmonic potential in the semiconductor. Both the GaAs/AlGaAs quantum well (QW) structures and SiGe/Si heterostructures are attempted, working with and without using a piezoelectric field in the semiconductor layer. The standing-wave fields generated in the semiconductor and the electron and hole distributions driven by the piezoelectric field are computed by finite element method (FEM) techniques. It is experimentally shown that, in a GaAs/Al_xGa_1-x As asymmetric double quantum well structure, the resonance enhancement of the narrower QW photoluminescence band is observed, which may be explained by the resonant charge transfer between the wider and narrower QWs. It is also shown that the piezoelectric fields quench the pure LO-phonon lines in the Raman spectra, whereas the coupled LO-phonon-plasmon mode strengthens. Experimental results indicate that the charge separation occurs in the plane of the QWs due to the piezoelectric fields. The recombination of carriers in the SiGe/Si heterostructures can be effectively enhanced by the presence of ultrasonic stress, displaying features consistent with varying electrical activity at dislocations.     

Picosecond luminescence of AlxGa1-xAs and GaAs double quantum well structures under high pressure

Abstract

The time-resolved luminescence of an electron-hole plasma in Al_0.36Ga_0.64As was studied as a function of pressure. Application of hydrostatic pressure varies the energy separation between the conduction band minima at Г and X. The dependence of the intensity ratio of the zero-phonon line and the two phonon replicas on this energy separation shows that scattering from X to Г by alloy disorder is as effective as scattering by phonons. We have further studied the tunneling of electrons between two GaAs quantum wells (QW) of different thicknesses through an Al_0.35Ga_0.65As barrier. The lifetime of electrons in the narrow QW, which is limitd by electron tunneling into the wider QW, stays constant from 0 to 2.4 GPa, where it drops within 0.1 GPa from 140 ps to less than 7 ps. At this pressure the X-point energy of the barrier coincides with the electron level in the wider QW. We infer that tunneling occurs only via the Г states in the barrier and that X states become effective only when real-state transfer is possible.     

Electron transport properties through double-barrier structures sandwiching a wide band-gap layer

We investigate the time-dependent transport properties of the quantum well (QW) with a wide band-gap material layer, where Al atoms doped in the middle GaAs QW. We find that the raised potential well bottom can affect the position of current hysteresis, current oscillation frequency and final steady-state mean current value. Moreover in this special structure, we find a negative differential conductance and only a current hysteresis region, the plateau structure of I–V curve found in the AlGaAs/GaAs/AlGaAs QW disappears.     

Optical and electrical properties of modulation-doped n and p-type Ga x In1-x N y As1-y /GaAs quantum wells for 1.3 μm laser applications

We present a comprehensive study of spectral photoluminescence (PL), photoconductivity and Hall mobility in undoped, n and p-type modulation-doped quantum wells of Ga_1-x In _x N _y As_1-y /GaAs with varying nitrogen concentration. We show that the increasing nitrogen composition red shifts the energy gap and this red shift is accompanied with a reduction of the 2D electron mobility in the quantum wells. True temperature dependence of the band gap, free from errors associated with nitrogen induced exciton trapping effects, is observed because in the modulation doped QW samples PL emission is dominated by band-to-band recombination and the S-shape temperature dependence is eliminated. Excellent fit to semi-experimental Varshni equation is obtained and the temperature dependence of the band gap in the linear regime (dE/dT) is tabulated as a function of nitrogen concentration and the type of dopant.     

Photoluminescence studies of confined states in AlGaAs/GaAs asymmetric quantum well

In the recombination spectra of AlGaAs/GaAs heterostructures, a peculiar and asymmetric photoluminescence (PL) band F has previously been reported [Aloulou et al., Mater. Sci. Eng. B 96 (2002) 14] to be due to recombinations of confined electrons from the two-dimensional electron gas (2DEG) formed at AlGaAs/GaAs interface in asymmetric quantum well (AQW). Detailed experiments are reported here on GaAs/Al_0.31Ga_0.69As/GaAs:δSi/Al_0.31Ga_0.69As/GaAs samples with different spacer layer thicknesses. We show that the band F is the superposition of two PL bands F′ and F″ associated, respectively, to AQW and a symmetric quantum well (SQW). In the low excitation regime, the F′ band present a blue shift (4.4 meV) followed by important red shift (16.5 meV) when increasing optical excitation intensity. The blue shift in energy is interpreted in terms of optical control of the 2DEG density in the AQW while the red shift is due to the narrowing of the band gaps caused by the local heating of the sample and band bending modification for relatively high-optical excitation intensity. Calculation performed using self-consistent resolution of the coupled Schrödinger–Poisson equations are included to support the interpretation of the experimental data.     

Intersubband absorption in highly photoexcited semiconductor quantum wells

Transient mid infrared (MIR) absorption spectroscopy is used to investigate transitions between higher electronic subbands in semiconductor quantum well (QW) structures after interband photoexcitation with intense picosecond pulses in the visible spectral range. Our investigation focuses on the e₂–e₃ intersubband transition in an asymmetric undoped GaAs/AlGaAs QW structure. At an injected nonequilibrium carrier density of 1×10¹³ cm⁻²/QW, an e₂–e₃ absorption band at 99 meV with a spectral width of 5 meV is found. For a higher density studied, 3×10¹³ cm⁻²/QW, the band is broadened and blueshifted by 30 meV. Intersubband absorption signals are distinguished from free-carrier absorption signals in the MIR by their characteristic time behavior.     

Valence bands spin splitting in GaAs/AlGaAs quantum wells

A quantitative comparison between different model calculations of valence band states in GaAs/AlGaAs heterostructures is presented. We demonstrate that a 14-band k.p Hamiltonian using a completely new parameterization based on fits of the tight-binding band structure leads to energy dispersion relations in excellent agreement with experiment, whereas previous parameterizations result in significant deviations. The relevance of the present results to the calculation of spin-related phenomena is discussed.     

Pressure studies of conduction-band N-pair-state mixing in dilute GaAs1−xNx alloys

High-pressure photoluminescence (PL) experiments (at 9 K) are reported for GaAs_1−xN_x/GaAs quantum wells having N compositions (x=0.0025, 0.004) in the dilute regime where the GaAs_1−xN_x alloy conduction band (CB) evolves rapidly by incorporation of N-pair states. Under increasing pressure, the PL spectra exhibit several new N-pair features that derive from CB-resonant states at 1 atm. Two of these features appear strongly at sub-band-gap energies for P29 kbar in the x=0.0025 sample, but are absent for all pressures in the x=0.004 sample. Several competing PL assignments due to bound-exciton recombination at NN_i pairs (i=1–4 is the anion separation) are considered in light of prior findings for N-doped (10¹⁷ cm⁻³) GaAs. The absence of certain PL features in the x=0.004 sample shows that N-pair states mix into the CB-continuum via a selective process, and this selectivity offers an important test for band-structure calculations in dilute GaAs_1−xN_x alloys.     

Photoreflectance study of InAs ultrathin layer embedded in Si-delta-doped GaAs/AlGaAs quantum wells

Photoreflectance and photoluminescence studies were performed to characterize InAs ultrathin layer embedded in Si-delta-doped GaAs/AlGaAs high electron mobility transistors. These structures were grown by Molecular Beam Epitaxy on (1 0 0) oriented GaAs substrates with different silicon-delta-doped layer densities. Interband energy transitions in the InAs ultrathin layer quantum well were observed below the GaAs band gap in the photoreflectance spectra, and assigned to electron-heavy-hole (E_e-hh) and electron-light-hole (E_e-lh) fundamental transitions. These transitions were shifted to lower energy with increasing silicon-δ-doping density. This effect is in good agreement with our theoretical results based on a self-consistent solution of the coupled Schrödinger and Poisson equations and was explained by increased escape of photogenerated carriers and enhanced Quantum Confined Stark Effect in the Si-delta-doped InAs/GaAs QW. In the photoreflectance spectra, not only the channel well interband energy transitions were observed, but also features associated with the GaAs and AlGaAs bulk layers located at about 1.427 and 1.8 eV, respectively. By analyzing the Franz-Keldysh Oscillations observed in the spectral characteristics of Si-δ-doped samples, we have determined the internal electric field introduced by ionized Si-δ-doped centers. We have observed an increase in the electric field in the InAs ultrathin layer with increasing silicon content. The results are explained in terms of doping dependent ionized impurities densities and surface charges.     

Optical properties of narrow high quality GaAs/AlGaAs quantum wells grown by MOVPE

We report optical characterization of high quality quantum well (QW) structures grown by metal-organic vapour-phase epitaxy (MOVPE). Thin QW layers of GaAs of thicknesses between 20 Å and 80 Å inserted between Al_0.36Ga_0.64 As confining layers as well as nominally 20 Å QW's in Al_xGa_1−xAs with varying x have been studied. Exciton confinement energies exceeding 250 meV and a FWHM of 6 meV for the thinnest QW have been observed. The photoluminescence (PL) data allows the observation of monolayer fluctuations in the QW widths and indicates an interface abruptness of about one atomic layer. Photoluminescence excitation spectroscopy allows electronic excited states to be seen.     

Nonselective vertical etching of GaAs and AlGaAs/GaAs in high pressure capacitively coupled BCl3/N2 plasmas

We studied nonselective, vertical dry etching of GaAs and AlGaAs/GaAs structure in high pressure capacitively coupled BCl₃/N₂ plasmas. The operating pressure was fixed at 150 m Torr. We found that there was an optimized process condition for nonselective and vertical etching of GaAs and AlGaAs/GaAs at the relatively high pressure. It was noted that there was a range of % N₂ (i.e. 20–40%) where nonselective etching of GaAs over AlGaAs could be achieved in the BCl₃/N₂ mixed plasma. We also found that dry etching of GaAs and AlGaAs/GaAs structure provided quite vertical and smooth surface when % N₂ was in the range of 0–20% in the BCl₃/N₂ plasma. The maximum etch rates for GaAs (0.41 μm/min) and AlGaAs/GaAs structure (0.42 μm/min) were obtained with 20–30% N₂ composition in the plasma.     

Investigation of various optical transitions in GaAs/Al0.3Ga0.7As double quantum ring grown by droplet epitaxy

This work examines the optical transitions of a GaAs double quantum ring (DQR) embedded in Al_0.3Ga_0.7As matrix by photoreflectance spectroscopy (PR). The GaAs DQR was grown by droplet epitaxy (DE). The optical properties of the DQR were investigated by excitation‐intensity and temperature‐dependent PR. The various optical transitions were observed in PR spectra, whereas the photoluminescence (PL) spectrum shows only the DQR and GaAs band emissions. The various optical transitions were identified for the GaAs near‐band‐edge transition, surface confined state (SCS), DQR confined state, wetting layer (WL), spin–orbital split (E_GaAs + Δ_o), and AlGaAs band transition. PR spectroscopy can identify various optical transitions that are invisible in PL. The PR results show that the GaAs/AlGaAs DQR has complex electronic structures due to the various interfaces resulting from DE.     

Optical properties of GaInNAsSb/GaAs/GaAs1−xNx (x ≈ 10%) saturable absorber quantum wells

We study the effect of the GaAsN narrow QWs on the optical properties of the GaInNAsSb/GaAs QWs using photoluminescence spectroscopy. A drastic effect of the N-rich layers on the QW photoluminesecnec (PL) intensity was observed with a strong influence of the spacer thickness. In the PL spectra a broad band caused by excitonic transitions related with N-related clusters in GaAs barriers is found. Based on calculations from experimental data, we have identified the low QW peak energy to the E₁-H₁ transition using the shear deformation potentials report Δp/p = 0.24.     

Role of hydrogen in improving optical quality of GaNAs alloys

Effects of hydrogen irradiation on optical quality of GaN_xAs_1−x alloys grown by gas source molecular beam epitaxy are studied by photoluminescence (PL) and PL excitation spectroscopies. A strong blue shift of the alloy band gap towards the value of a reference N-free GaAs samples is revealed. Substantial improvements in the optical quality of the structures induced by hydrogen, due to a reduction of band edge potential fluctuations, is also demonstrated. The effect is tentatively attributed to the preferential trapping of hydrogen atoms at the lattice sites with the highest N compositions, which leads to the neutralization of electronic states contributing to the band tail states as well as to the improvement in strain uniformity.     

Identification and elimination of inductively coupled plasma-induced defects in AlxGa1 - xN/GaN heterostructures

By using temperature-dependent Hall,variable-frequency capacitance-voltage and cathodoluminescence (CL) measurements,the identification of inductively coupled plasma (ICP)-induced defect states around the Al x Ga 1-x N/GaN heterointerface and their elimination by subsequent annealing in Al x Ga 1-x N/GaN heterostructures are systematically investigated.The energy levels of interface states with activation energies in a range from 0.211 to 0.253 eV below the conduction band of GaN are observed.The interface state density after the ICP-etching process is as high as 2.75×10 12 cm 2 ·eV 1.The ICP-induced interface states could be reduced by two orders of magnitude by subsequent annealing in N 2 ambient.The CL studies indicate that the ICP-induced defects should be Ga-vacancy related.     

Defects and localized states in MBE-grown GaAs1−x N x solid solutions prepared by molecular-beam epitaxy

Defects and localized states have been studied for molecular-beam-epitaxy (MBE)-grown high-resistivity and undoped GaAs_1?x N _x films with a N concentration not exceeding approximately 1.0 at.%. The crystalline quality of the films and hence the defects and localized states were determined by high-resolution X-ray diffraction, photoluminescence spectra, capacitance versus voltage measurements and photoinduced current transient spectra of GaAs and GaAs_1?x N _x layers. It was concluded that incorporation of low concentrations of N into MBE-grown GaAs_1?x N _x films promotes the formation of high densities of deep centres similar to EL2 donors, leading to heavy compensation of the films by some unidentified acceptors. GaAs antisite acceptors were believed to be responsible for the said compensation. A prominent defect band near 1.33-1.38 eV also appeared to be associated with these defects. The most prominent centres in dilute GaAs_1?x N _x films with N content less than 0.35 at.% seem to be the EL2 donors and the hole traps located near E _v +0.3 eV.     

Visible photoluminescence in porous GaAs capped by GaAs

A typical porous structure with pores diameters ranging from 10 to 50 nm has been obtained by electrochemical etching of (1 0 0) heavily doped p-type GaAs substrate in HF solution. Room temperature photoluminescence (PL) investigations of the porous GaAs (π-GaAs) reveal the presence of two PL bands, I₁ and I₂, located at 1.403 and 1.877 eV, respectively. After GaAs capping, the I₁ and I₂ PL bands exhibit opposite shift trends. However, the emission efficiency of these two bands is not strongly modified. Low temperature PL of capped porous GaAs versus injection levels shows that the I₁ PL band exhibits a red shift while the I₂ PL band exhibits a blue shift with increasing injection levels. The I₂ PL band intensity temperature dependence shows an anomalous behaviour and its energy location shows a blue shift as temperature increases. The observed PL bands act independently and are attributed to electron – hole recombination in porous GaAs and to the well-known quantum confinement effects in GaAs nanocrystallites. The I₂ PL band excitation power and temperature dependencies were explained by the filling effect of GaAs nanocrystallites energy states.     

Optical characterization of InGaAsN/GaAsN/GaAs quantum wells with InGaP cladding layers

Optical properties of InGaAsN/GaAs and InGaAsN/GaAsN/GaAs quantum well structures with InGaP cladding layers were studied by photoreflectance at various temperatures. The excitonic interband transitions of the InGaAsN/GaAsN/GaAs QW systems were observed in the spectral range above hν=E_g(InGaAsN). The confinement potential of the system with strain compensating GaAsN barriers became one step broader, thus more quantum states and larger optical transition rate were observed. A matrix transfer algorithm was used to calculate the subband energies numerically. Band gap energies, effective masses were adopted from the band anti-crossing model with band-offset values adjusted to obtain the subband energies to best fit the observed optical transition features. A spectral feature below and near the GaAs band gap energy from GaAs barriers is enhanced by the GaAs/InGaP interface space charge accumulation induced internal field.     

Nature of the eigenstates in the miniband of random dimer-barrier superlattices

The dc conductance, the universal quantum fluctuations and the resistance distribution are numerically investigated in dimer semiconductor superlattices by means of the transfer matrix formalism. We are interested in the GaAs/Al_x Ga _1 − xAs layers, having identical thickness, where the aluminium concentration x takes, at random, two different values, with the constraint that one of them appears only in pairs, i.e. the random dimer barrier (RDB). These systems exhibit a miniband of extended states, around a critical energy, lying to the typical structure of the dimer cell. The states close to this resonant energy consist of weakly localized states, while in band tails i.e. for negligible conductance, the states are strongly localized. This is evidence of the suppression of localization in the RDB superlattices. The nature of the transition between these two regimes is quantitatively investigated through relevant physical quantities. The model is, hence, clearly and statistically examined.     

Bandgap tuning of semiconductor quantum well structures using ion implantation

Ion induced QW intermixing using broad area and focused ion beam (FIB) implantation was investigated at low energy (32 and 100 keV respectively) in three different material systems (GaAs/AlGaAs, InGaAs/GaAs, and lattice matched InGaAs/InP). Repeated sequential ion implants and rapid thermal anneals (RTAs) were successful in delivering several times the maximum QW bandgap shift achievable by a single implant/RTA cycle. The effectiveness of broad area high energy implantation (8 MeV As⁴⁺) on QW intermixing was also established for GRINSCH (graded-index separate confinement heterostructure) QW laser structures grown in InGaAs/GaAs. Lastly, preliminary work illustrating the effects of implant temperature and ion current density was carried out for InGaAs/GaAsQWs.