Δ-doped GaAs Layers Research Articles

Photoluminescence characterization of nonradiative recombination centers in MOVPE grown GaAs:N δ-doped superlattice structure

Nonradiative recombination (NRR) centers in GaAs:N δ-doped superlattice (SL) structure grown by metal organic vapor phase epitaxy (MOVPE) are studied by two-wavelength-excited photoluminescence (PL) method. The PL intensity quenching due to the superposition of below-gap excitation (BGE) light at energies of 0.75, 0.80, 0.92, and 0.95 eV and an above-gap excitation (AGE) light either for the GaAs conduction band (1.69 eV) or E− band excitation (1.45 eV), respectively indicates the existence of NRR centers inside the GaAs:N δ-doped SL and GaAs layers. The AGE density and BGE density dependence of the PL intensity have been studied for both excitation schemes. Recombination model based on two levels has been proposed to interpret the experimental results and energy distribution of NRR centers inside the GaAs:N δ-doped SLs and GaAs layers. The defect-related parameters are estimated systematically by solving the rate equations and fitting the results with the experimental data that give a phenomenological insight into the sample.

Nonradiative recombination centers in GaAs:N δ-doped superlattice revealed by two-wavelength-excited photoluminescence

We use two-wavelength-excited photoluminescence (PL) to investigate nonradiative recombination (NRR) centers in GaAs:N δ-doped superlattice (SL) structures grown by molecular beam epitaxy. The change in photoluminescence (PL) intensity due to the superposition of below-gap excitation at energies of 0.75, 0.80, 0.92, and 0.95 eV and above-gap excitation at energies of 1.69 or 1.45 eV into the GaAs conduction band and the E− band implies the presence of NRR centers inside the GaAs:N δ-doped SL and/or GaAs layers. The change in PL intensity as a function of the photon number density of below-gap excitation is examined for both bands, which enables us to determine the distribution of NRR centers inside the GaAs:N δ-doped SL and GaAs layers. We propose recombination models to explain the experimental results. Defect-related parameters that give a qualitative insight into the samples are investigated systematically by fitting the rate equations to the experimental data.

Effects of rapid thermal annealing on two-dimensional delocalized electronic states of the epitaxial N δ-doped layer in GaAs

We have conducted rapid thermal annealing (RTA) for improving the two-dimensional (2D) arrangement of electronic states in the epitaxial nitrogen (N) δ-doped layer in GaAs. RTA rearranged the N-pair configurations in the GaAs (001) plane and reduced the number of non-radiative recombination centers. Furthermore, a Landau shift, representing the 2D delocalized electronic states in the (001) plane, was observed at around zero magnetic field intensity in the Faraday configuration.

Dopant mapping of Be δ-doped layers in GaAs tailored by counterdoping using scanning tunneling microscopy

The effect of counterdoping on the Be dopant distribution in delta (δ)-doped layers embedded in Si-doped and intrinsic GaAs is investigated by cross-sectional scanning tunneling microscopy. δ-doped layers in intrinsic GaAs exhibit a large spreading, whereas those surrounded by Si-doped GaAs remain spatially localized. The different spreading is explained by the Fermi-level pinning at the growth surface, which leads to an increased Ga vacancies concentration with increasing Si counterdoping. The Ga vacancies act as sinks for the diffusing Be dopant atoms, hence retarding the spreading.

Effect of the Doping Layer Concentration on Optical Absorption in Si δ-Doped GaAs Layer

We study in this paper the intersubband optical absorption of Si doped GaAs layer for different applied electric fields and donors concentration. The electronic structure has been calculated by solving the Schr?dinger and Poisson equations self-consistently. From our results, it is clear that the subband energies and intersubband optical absorption are quite sensitive to the applied electric field. Also our results indicate that the optical absorption depends not only on the electric field but also on the donor’s concentration. The results of this work should provide useful guidance for the design of optically pumped quantum well lasers and quantum well infrared photo detectors (QWIPs).

Voltage-controlled electron tunneling from a single self-assembled quantum dot embedded in a two-dimensional-electron-gas-based photovoltaic cell

We perform high-resolution photocurrent (PC) spectroscopy to investigate resonantly the neutral exciton ground-state (X0) in a single InAs/GaAs self-assembled quantum dot (QD) embedded in the intrinsic region of an n-i-Schottky photodiode based on a two-dimensional electron gas (2DEG), which was formed from a Si δ-doped GaAs layer. Using such a device, a single-QD PC spectrum of X0 is measured by sweeping the bias-dependent X0 transition energy through that of a fixed narrow-bandwidth laser via the quantum-confined Stark effect (QCSE). By repeating such a measurement for a series of laser energies, a precise relationship between the X0 transition energy and bias voltage is then obtained. Taking into account power broadening of the X0 absorption peak, this allows for high-resolution measurements of the X0 homogeneous linewidth and, hence, the electron tunneling rate. The electron tunneling rate is measured as a function of the vertical electric field and described accurately by a theoretical model, yielding information about the electron confinement energy and QD height. We demonstrate that our devices can operate as 2DEG-based QD photovoltaic cells and conclude by proposing two optical spintronic devices that are now feasible.

Bias-controlled single-electron charging of a self-assembled quantum dot in a two-dimensional-electron-gas-basedn−i-Schottky diode

(Received 9 December 2010; revised manuscript received 12 January 2011; published 14 February 2011) We present bias-dependent micro-photoluminescence (μ-PL) spectroscopy of the neutral (X 0 ) and singly negatively-charged (X − ) excitons in single InAs/GaAs self-assembled quantum dots (QDs) embedded in the intrinsic region of an n-i-Schottky diode based on a two-dimensional electron gas (2DEG), which was obtained from a Si δ-doped GaAs layer. Using such a device structure, we demonstrate bias-controlled single-electron charging of a single QD as the QD s-shell electron state is tuned below the Fermi level. This is verified experimentally by the sequential appearance of energetically-distinct PL emission lines from the two excitons and supported by theoretical calculations. In addition, it is shown both experimentally and theoretically that simultaneous PL emission from the X 0 and X − excitons within a particular bias range is the result of a long-lived charge-nonequilibrium state due to weak tunnel-coupling between the QDs and 2DEG in our device. Further, the ability to tune the exciton transition energies via the quantum-confined Stark effect is observed, offering insight into the carrier wave function distributions in the QD and the QD material structure. Finally, we propose a number of spintronic device concepts that may be made feasible as a result of this investigation into bias-controlled carrier tunneling between a self-assembled QD and a 2DEG.

Effect of Magnetic Field on a p-Type δ-Doped GaAs Layer

The energy levels of holes in a p-type δ-doped GaAs structure under a magnetic field are theoretically calculated within the framework of the effective mass approximation for a uniform acceptor distribution. The electronic structure is calculated by solving the Schrödinger and Poisson equations self-consistently. The effect of the magnetic field on the potential profile changes the degree of the confinement and localization, and thus this behavior can be used to study these systems in regions of interest, without the need to grow many different samples. It is found that the heavy-hole subbands contain many more energy states than the light-hole ones; the population of the heavy-hole levels represents approximately 91% of all the carriers without magnetic field. With increasing magnetic field the total population of the heavy-holes increases and the number of filled states changes.

Photoluminescence from single isoelectronic traps in nitrogen delta-doped GaAs grown on GaAs(1 1 1)A

We have studied photoluminescence (PL) observed from single isoelectronic traps formed by nitrogen pairs in nitrogen δ-doped GaAs layers grown on GaAs(1 1 1)A substrates. The PL was composed of a single peak with a narrow linewidth of ∼80 μeV. Polarized PL measurements confirmed that the emission from single isoelectronic traps in nitrogen δ-doped GaAs(1 1 1) is unpolarized irrespective of nitrogen pair arrangements. These results can be explained by in-plane isotropy of the samples, which is consistent with the symmetrical property of GaAs(1 1 1), and demonstrate that utilizing (1 1 1) substrate is an effective means for obtaining unpolarized single photons, which are desirable for the application to quantum cryptography.

Crystalline and electrical characteristics of C 60-doped GaAs films

C 60 uniformly doped and δ-doped GaAs layers are grown by migration enhanced epitaxy method. Crystalline and electrical characteristics of the layers are investigated by reflection high energy electron diffraction, X-ray diffraction and capacitance–voltage ( C– V) measurements. C 60 high-concentration doping is found to introduce 2D defects, and X-ray diffraction pole-figure measurements show that the rotational domains appear predominantly on {1 1 1}A plane. This indicates that C 60 molecules are mainly incorporated on the dangling bonds of Ga atoms due to the high binding energy between C 60 molecules and Ga atoms. C 60 uniformly doped GaAs layers show highly resistive characteristics, suggesting that C 60 molecules cannot be decomposed into isolated C atoms in the GaAs lattice, and they behave as if they were electron traps or strong recombination centers. C– V profiles of Cr–Au/C 60 δ-doped GaAs Schottky diode suggest that C 60 molecules in GaAs lattice produce electron traps which can be charged or discharged by applied electrical field.

P-Type δ-Doped Layers in GaAs Isolated by He+ Bombardment

Ion implantation is an essential process for production of modern III-V compound semiconductor devices and circuits and has been proven as a successful method to convert a conductive layer into a highly resistive one. Due to its simplicity, precise depth control and compatibility with planar technologies, ion implantation is a potential alternative for mesa etching. Selective masking of the semiconductor surface with photoresist followed by ion irradiation is an efficient and practical way to isolate closely spaced devices. Irradiation of GaAs by ions results in resistivity increase as a consequence of free carrier removal by trapping at defects introduced by the irradiation. Delta-doping is a well known technique which spatially confines the dopants to one atomic layer during epitaxial growth of semiconductor materials. In the present work a dose dependence of sheet resistance of delta- doped GaAs structures, irradiated by He+ ions, was inve stigated. It was found that a threshold dose for electrical isolation of the structures was determined by the energy of ions and by the concentration of initial doping.

Isolation of p-type carbon δ-doped layers in GaAs by ion bombardment

The evolution of the sheet resistance (Rs) in δ-doped layers in GaAs during He ion irradiation was investigated. The Rs increases with the dose accumulation in a similar way that occurs in layers with much wider dopant profiles, produced either by ion implantation or epi-growth. The Rs becomes higher than 109 Ω/sq after the so-called threshold dose (Dth) is accumulated. The values of Dth closely correlate with the ratio of the estimated number of replacement collisions at the depth of the δ-doped layer and the original sheet hole concentration. The maximum thermal stability of the isolation, i.e. the persistence of Rs at values above 109 Ω/sq after a subsequent thermal annealing is of about 550–600 °C.

Determination of the doping concentration profile in Si δ-doped GaAs layers using micro-Raman spectroscopy of bevelled structures

We present a new method of determining the depth profile of the doping concentration in Si δ-doped GaAs layers and GaAs-based heterostructures by micro-Raman spectroscopy. In-depth resolution is obtained by scanning the laser spot along a shallow bevel synthesized in the structure of interest. The doping level is determined by assessing changes in the ratio of the transversal to the longitudinal optical phonon lines intensities. The determined doping concentration values are in good agreement with those determined by secondary ion mass spectroscopy and capacitance–voltage methods.

Study of δ-doped GaAs layers by micro-Raman spectroscopy on bevelled samples

Si δ-doped GaAs layers were studied by micro-Raman spectroscopy. The spectra were recorded along the bevelled structure using the light of Ar +-ion laser (514.5 nm line) with high power density. The observed changes in the Raman spectra are discussed in the sense of coupling present between LO phonon and photoexcited electron–hole plasma and plasma of electrons arising from ionised Si atoms. Plasmon-LO-Phonon modes of the coupling of photoexcited plasma in δ-doped GaAs layers were observed for the first time. The minimal thickness of cap layer was estimated in the range of 10–19 nm depending on the doping concentration.

Intersubband optical absorption of double Si δ-doped GaAs layers

For different applied electric fields the intersubband transitions of double Si δ-doped GaAs structures are theoretically investigated for a uniform donor distribution. The electronic structure has been calculated by solving the Schrödinger and Poisson equations self-consistently. It is found that the changes of the confining potential, the subband energies and intersubband optical absorption are quite sensitive to the applied electric field. Different from the single δ-doped structure, we have seen the abrupt changing of the subband energies and the absorption coefficient whenever the applied electric field reaches two certain values. While for the (1–2) intersubband transition, the intersubband absorption spectrum shows blueshifts up to the first critical electric field value, this spectrum shows redshifts up to the second critical electric field value and shows blueshifts for higher electric fields than the second critical field values. This behavior in the intersubband transitions for different electric fields can be used in some infrared optical device applications based on the intersubband transitions of electrons.

Intersubband transitions for single, double and triple Si δ-doped GaAs layers

The intersubband transitions in single, double and triple Si δ-doped GaAs structures are theoretically studied for different applied electric fields. Electronic properties such as the confining potential, the subband energies and the eigen envelope wave functions have been calculated by solving the Schrödinger and Poisson equations self-consistently. We have seen that the intersubband transitions are very sensitive to the type of structure and are dependent on the applied electric field. The electric field and structure dependence of the intersubband optical absorption is interesting for potential device applications in a class of photodetectors and optical modulators.

The electric field effects on intersubband optical absorption of Si δ-doped GaAs layer

The intersubband transitions in Si δ-doped GaAs structures is theoretically investigated for different applied electric fields. For an uniform distribution the electronic structure has been calculated by solving the Schrödinger and Poisson equations self-consistently. From our calculations, it is found that the subband energies and intersubband optical absorption is quite sensitive to the applied electric field. This gives a new degree of freedom in various device applications based on the intersubband transitions of electrons.

MBE growth, structural, and transport properties of Mn δ-doped GaAs Layers

We have grown Mn δ-doped GaAs layers by molecular beam epitaxy with various Mn sheet concentrations ( θ Mn) defined in the monolayer (ML) unit. Structural analyses using cross-sectional transmission electron microscopy and secondary ion mass spectroscopy revealed that when θ Mn is below 1 ML, Mn dopants are abruptly confined within 2–3 ML without lattice dislocations and clusters, and also without diffusion and surface segregation. Characterizations on the thermal stability of the doping profiles showed that the profiles retained abruptness even at elevated growth temperature T s=400°C without diffusion and significant segregation. Strong surface segregation of Mn atoms, lattice dislocations, and MnAs clusters appeared when θ Mn exceeds 1 ML. The transport properties of Mn δ-doped GaAs layers without and with selectively p-type doping are also presented.

The effect of the donor distribution on the electronic structure of two coupled Si δ-doped layers in GaAs

We have theoretically investigated the electronic structure of two coupled Si δ-doped GaAs at T=0 K. For the uniform donor distribution we have studied the influence of donor concentration on the subband structure. In order to obtain the electronic structure we have calculated self-consistently the Schrödinger and Poisson equations. From the self-consistent calculation, we have seen that the electronic structure is quite sensitive to the donor concentration and the δ-layer separation. Under the assumption that the mobility is limited by the overlap of the wave function and the ionized scattering centers we can estimate that the mobility in single δ-doped structures is very low compared to closely spaced coupled δ-doped GaAs structures.

Micro‐photoluminescence on bevelled samples – a method of assessing δ‐doped layer

This paper reports a method for the determination of doping concentration profile in single and multi δ-doped GaAs layers on bevelled samples. The method is based on the recording and evaluation of micro-photoluminescence spectra along the bevelled structure. The values for the doping concentration profiles are in good agreement with those obtained by C–V, SIMS and Raman spectroscopy.