Δ-doped Structures Research Articles

Influence of partial dopant ionization on the capacitance–voltage profiles of delta-doped structures

The effects of partial dopant ionization on the capacitance–voltage (C–V) characteristics of δ-doped structures have been investigated using self-consistent simulations. The simulation results show that partially ionized δ-doped dopants should produce a much sharper C–V profile compared with the case of fully ionized dopants. The results reveal also that the main factor which determines the spatial resolution and the full width at half maximum of the C–V peak is the spatial extent of the dopant profile rather than the spatial extent of the ground-state wave function. From this, it is suggested that the δ-doped C–V data should be interpreted always with the effect of partial ionization of δ-doped dopants.

Simulation and analysis of the capacitance–voltage characteristics of the δ-doped semiconductors

In this paper, a self-consistent model to simulate the general characteristics of one-dimensional semiconductor structures is demonstrated. During calculation, possible quantum effects and the distributions of both electrons and holes are all considered. In this model, a continuity equation is solved to calculate the distribution of free electrons and holes. The possible quantum wells are sought using the Schrödinger equation. The overall charge density and potential are obtained self-consistently by an iteration scheme. The C– V characteristics of the δ-doped structures are simulated and then compared with those of practical samples. By comparing with these δ-doped samples, the effective numbers of dopant atoms can be precisely determined. For these highly doped samples, it is found that the activation rates are only about half. This finding can be verified by Hall measurements which confirms the accuracy in this study.

A new C δ-doping precursor

Low dimensional doping (usually known as S-doping) is a technique which intends to spatially confine the dopants to one or a few atomic layers during epitaxial growth of semiconductor materials. Extensive studies of S-doping in III-Vs arise from unique electronic and optoelectronic properties of δ-doped structures and practical flexibility of placing S-doped layers into novel devices for improved performance. Recent progress has led to successful growth of n-type and p-type δ-doped III-Vs by MOCVD.

Impurity resistivity in doubly δ-doped systems

We present an expression for the impurity resistivity for a general number of subbands in singly and doubly δ-doped structures. The true wave functions of the carriers and the intraband scattering from interaction between carriers of different bands are considered. The calculations are performed within the generalized Drude approach and with the temperature dependent RPA screening matrix. Numerical results for general temperatures are presented for singly n-doped Si and doubly n-doped Si with varying distance between the doped layers. We propose an explanation for the observed enhancement of the mobility in doubly δ-doped structures, as compared to singly δ-doped systems.

Comparison of OMVPE and MBE grown AlGaAs/InGaAs PHEMT structures

δ-doped PHEMT structures commercially grown by OMVPE and MBE on 3 inch wafers were examined using contactless resistance, magneto-Hall, PR, PL, DXRD, RBS, SIMS, and electrochemical capacitance-voltage measurements, and 0.1 μm gate length devices were fabricated from them and then characterized. The electron mobilities in the OMVPE 2DEG were a little smaller than those in the MBE 2DEG, but were still excellent, and the carrier concentrations, which were > 2.5 × 1012cm−2 at 77 K, were similar. The interface quality as measured by PR spectra was a little better in the MBE sample. Variation in the In areal density as determined by RBS was a little larger in the OMVPE wafer, but it was still less than 1 at%. The sheet resistance and the doping concentration as determined by SIMS and EC-V measurements showed the variation in the OMVPE wafers was slightly larger, but it was still considered to be small. PHEMTs fabricated from the most promising OMVPE wafer had gm's (753 mS/mm) that approached those for the MBE wafer (796 mS/mm) as did their fT's (115 versus 117 Ghz). The variations in these parameters were also found to be slightly larger in the OMVPE wafers.

Anisotropy of the conductivity in δ-doped multilayers

We have investigated three Si δ-doped multilayer structures with sheet-doping concentrations around N D = 5 × 10 11cm −2 and sheet distances of 17, 40 and 100 nm. Cyclotron resonance measurements and far-infrared spectroscopy in a multiple reflection geometry allow the determination of the effective masses and the scattering times normal and parallel to the doping planes. While the scattering time was found to be isotropic, the effective mass exhibited a strong anisotropy with m ⊥ < m ∥. The measurements are supported by a self-consistent calculation of the subband structure of the samples.

Quenching of photoconductivity by a strong electric field in tin δ-doped GaAs structures

The conductivity of GaAs structures δ-doped with tin on the vicinal and singular faces was investigated in strong electric fields up to E=104 V/cm and temperatures in the range 4.2 K <T<300 K. The measurements were performed in the dark and under illumination with visible light. Long-time photoconductivity of 2D electrons with threshold T c ≈240 K was observed in samples which were δ-doped with tin on the vicinal face. A strong electric field not only quenches photoconductivity, but also increases the resistance of the structures at temperatures T<T c by several orders of magnitude with respect to the dark resistance.

Suppression of deformation-potential electron-acoustic-phonon coupling in Si delta -doped GaAs structures.

We have used the heat-pulse technique to investigate phonon emission from two-dimensional electron gases formed in a series of molecular-beam-epitaxy- and metal-organic chemical-vapor-deposition-grown Si \ensuremath{\delta}-doped GaAs structures with single subband occupation. By comparing the relative intensities of longitudinal and transverse acoustic-phonon emission over a wide range of input powers (0.01--1000 pW per electron) we suggest that deformation-potential coupling to longitudinal modes is suppressed in the acoustic regime (${\mathit{T}}_{\mathit{e}}$50 K). In all samples the onset of optical-phonon emission was at input powers around 1.5 pW per electron, a value similar to that found in the heterojunction and quantum-well cases. Strong LA-mode emission is observed at all powers in contrast to previous observations in the heterojunction system.

Comparison of OMVPE Grown GaAs/AlGaAs and GaAs/InGaP HEMT and PHEMT structures

Symmetric δ-doped InGaP and AlGaAs PHEMT structures have been grown by organometallic vapor phase epitaxy with properties that approach those of MBE grown AlGaAs structures. The 300 and 77K carrier concentrations for the InGaP PHEMT were 2.72 and 2.56 × 1012 cm2−2 and the mobilities were 5,920 and 22,000 cm22/V.s. These excellent values suggest that problems associated with switching the anion at the channel heterojunction have been overcome. The corresponding values for the AlGaAs PHEMT were 2.51 and 2.19 × 1012 cm2−2 and 6,500 and 20,400 cm2/V.s. The uniformity in the indium concentration in the InGaAs layer as determined by photoluminescence, photoreflection, double crystal x-ray diffraction, and Rutherford backscattering was found to be good, but the percent In in the AlGaAs pseudo-morphic high electron mobility transistor (PHEMT) was less than that in the InGaP PHEMT even though the programmed values were the same. The uniformity in the doping distribution as determined by secondary ion mass spectroscopy and electrochemical capacitance-voltage measurements was found to be good, but it decreased with distance from the center of the susceptor. Also, most of the dopants in the δ-doped InGaP and AlGaAs layers were activated.

Time-resolved study of carrier capture and recombination in monolayer Be δ-doped GaAs

The first measurements of subpicosecond dynamics of carrier capture and decay in Be δ-doped GaAs structures with p-doping density ranging from 6×1012 to 2×1014 cm−2 and spatial distribution on monolayer scale show that photoexcited carriers are captured in the δ-doped layer in &amp;lt;1 ps. Luminescence decay rates show a strong dependence on energy of the hole in the two-dimensional Fermi gas. We attribute this to a change in the spatial electron-hole overlap resulting from band-mixing effects. We show that Be density fluctuations lead to strong carrier localization.

A scanning tunnelling microscopy study of the deposition of Si on GaAs(001); implications for Si δ-doping

Atomic resolution scanning tunnelling microscopy (STM) has been used to study the adsorption of Si on GaAs(001) surfaces, grown in situ by molecular beam epitaxy (MBE), with a view to understanding the incorporation of Si in δ-doped GaAs structures. Under the low-temperature deposition conditions chosen, the clean GaAs surface is characterized by a well-defined c(4 × 4) reflection high-energy electron diffraction (RHEED) pattern, a structure involving termination with two layers of As. Filled states STM images of this surface indicate that the basic structural unit, when complete, consists of rectangular blocks of six As atoms with the AsAs bond in the surface layer aligned along the [110] direction. Deposition of < 0.05 ML of Si at 400°C onto this surface shows significant disruption of the underlying structure. A series of dimer rows are formed on the surface which, with increasing coverage, form anisotropic “needle-like” islands which show no tendency to coalesce even at relatively high coverages (∼ 0.5 ML). The formation of these islands accompanies the splitting of the 1 2 order rods in the RHEED pattern along [110]. As the Si is known to occupy only Ga sites, the Si atoms displace the top layer As atoms of the c(4 × 4) structure, with the displaced As atoms forming dimers in a new top layer. The results are consistent with a recently proposed site exchange model and subsequent island formation for surfactant mediated epitaxial growth.

A scanning tunnelling microscopy study of the deposition of Si on GaAs(001); Implications for Si δ-doping

Atomic resolution scanning tunnelling microscopy (STM) has been used to study the adsorption of Si on GaAs(001) surfaces, grown in situ by molecular beam epitaxy (MBE), with a view to understanding the incorporation of Si in δ-doped GaAs structures. Under the low-temperature deposition conditions chosen, the clean GaAs surface is characterized by a well-defined c (4 × 4) reflection high-energy electron diffraction (RHEED) pattern, a structure involving termination with two layers of As. Filled states STM images of this surface indicate that the basic structural unit, when complete, consists of rectangular blocks of six As atoms with the AsAs bond in the surface layer aligned along the [110] direction. Deposition of < 0.05 ML of Si at 400°C onto this surface shows significant disruption of the underlying structure. A series of dimer rows are formed on the surface which, with increasing coverage, form anisotropic “needle-like” islands which show no tendency to coalesce even at relatively high coverages (∼ 0.5 ML ). The formation of these islands accompanies the splitting of the 1 2 order rods in the RHEED pattern along [110]. As the Si is known to occupy only Ga sites, the Si atoms displace the top layer As atoms of the c (4 × 4) structure, with the displaced As atoms forming dimers in a new top layer. The results are consistent with a recently proposed site exchange model and subsequent island formation for surfactant mediated epitaxial growth.

Effects of cation diffusion during chemical beam etching

The surface reconstruction during chemical beam etching of GaAs by AsCl3 is discussed. The existence of reflection high energy electron diffraction (RHEED) intensity oscillation indicates a planar etching mode in the initial stage. Its evolution into a three-dimensional (3D) etching can be understood in terms of suppressed cation diffusion during etch. Appropriate choice of etch parameters that enhance the cation diffusion is the key to obtain smooth etching morphology. The effectiveness of etch cleaning is subjected to the planarity of the surface during etch, and the chemical reactivity of the contaminants with the etching gas. This is illustrated by etching of the Be and Si δ-doped structures in GaAs.

Effects of cation diffusion during chemical beam etching

The surface reconstruction during chemical beam etching of GaAs by AsCl 3 is discussed. The existence of reflection high energy electron diffraction (RHEED) intensity oscillation indicates a planar etching mode in the initial stage. Its evolution into a three-dimensional (3D) etching can be understood in terms of suppressed cation diffusion during etch. Appropriate choice of etch parameters that enhance the cation diffusion is the key to obtain smooth etching morphology. The effectiveness of etch cleaning is subjected to the planarity of the surface during etch, and the chemical reactivity of the contaminants with the etching gas. This is illustrated by etching of the Be and Si δ-doped structures in GaAs.

Electron mobility enhancement in Si using doubly δ-doped layers

Large enhancements in the electron mobility are reported for structures containing a pair of closely spaced Sb δ-doped layers in Si. The room-temperature mobility is enhanced by a factor of 2 compared to corresponding uniformly doped layers of singly δ-doped structures. Even higher mobilities were obtained by using a Schottky gate on top and applying a voltage to adjust the potential well. With an effective gate voltage of ∼−0.3 V the mobility was 1200 cm2 V−1 s−1 at room temperature, which is an enhancement by a factor of 10 relative to the layer with equivalent bulk doping concentration. The high mobility is attributed to wave functions with nodes at the δ-doped layers.

Raman study of electron confinement in Si δ-doped GaAs

In this letter Raman spectroscopy measurements on Si δ-doped GaAs are reported. The coupling of two-dimensional (2D) intersubband plasmons with the polar phonons is clearly demonstrated. The change undergone by the coupled mode signal as a function of the depth explored by the Raman probe is studied and allows for an original and accurate investigation of electron gas confinement. Indeed, it is shown that a quantitative analysis of depth profiling measurements allows the real-space extent of the quasi-2D electron gas in δ-doped structures to be estimated.

Field emission from structures with quantum wells

Computer simulation studies of the properties of some novel types of field emitters have been carried out in the framework of the theory of quantum disc effects. The following cases have been analyzed in detail. (1) Emission from the field surface states (SS) exhibiting both discrete and continuous energy spectra. This case resembles the known effect of the work-function deterioration ΔW∼(Ec−ESS), yet, here, prior to the emission into vacuum, the SS electrons must be excited the conductivity band via the Pool–Frenkel mechanism: ne≂C exp(aEs1/2/kT), or some other. (2) Emission from the δ-doped structures via the internal field ionization of filled states, deposited near to surface in semiconductor or in super-thin insulator layer. (3) Emission from the two-dimensional (2D) quantum well (QW) surface space charge regions. In this case, the bottom of a subsurface c band is shifted by the quantization value E=Ec−Ei with Ei=(ℏ2/2m*)1/3 [3/2πeEs(i+3/4)]2/3∼Es2/3. Here Es=4π/2eNs is the strength of the surface electric field and i stands for the number of a quantum well (or a 2D subband). In contrast with the three-dimensional (3D) case, the electron concentration in the ith 2D degenerate subband is given bt a step function Ns=(m*/πℏ2)kT ln{1+exp[(Ec−Ei)/kT]}. With m*=0.01 me, the difference E=(Ei−Ec) greatly increases in pretty high applied fields (Es∼10 V cm−1) and, thus, may give rise to a pronounced increase of the field emission current. Moreover, with growing Es, subsequent bands are being filed and, due to the steplike dependence of NSS, an additional growth of the emission current may occur, together with emerging collisions on the Fowler–Nordheim differential curve. (4) Emission from very sharp tips containing 2D-conducting layer (QW space charge), when τ tip is comparable to the low thickness of QW layer. Here we have so-called zero-size (zero-dimensional) systems, when the dependence ΔW on ES becomes even sharper. (%) emission from the tips covered by very thin films with quantized conducting electrons. For these, narrow-band or semimetallic materials (such as Bi) with very low effective mass (HgCdTe, InSb, PbSTe, etc.) may be used. Discrete Q levels Ei govern the current oscillations with the increasing Es, when Ei is filled by the applied field. For the transversal current (in the direction of the electron localization) to appear, necessary determine or ionization 2D quasilocalized electron to more high, 3D subbands, or due to different scattering processes (including oscillation in QW).

Electronic properties of multiple Si δ-doped GaAs layers grown by molecular beam epitaxy and migration-enhanced epitaxy

We report on comparative measurements of the electronic properties of multiple Si δ-doped GaAs structures grown by conventional molecular beam epitaxy (MBE) and by migration-enhanced epitaxy (MEE) at a substrate temperature of 540°C. Samples were studied by Shubnikov-De Haas and Hall measurements. Results suggest that for multiple δ-doped samples grown at high substrate temperatures, MBE is a more effective growth technique.

Tunneling spectroscopy and miniband structure of selenium delta-doped GaAs

We have investigated the subband energies on Se δ-doped GaAs structures by tunneling current. Self-consistent calculations including many-body effects were performed to compare the experimental data. It was shown that increasing the gate voltage induces ionization of resonant deep donor. Carrier distribution was determined by capacitance-voltage measurements. The subsequent annealing experiments revealed that significant diffusion of the Se dopant at 700 ° C had taken place in comparison to the Si dopant case.

Fermi edge singularity and screening effects in the luminescence spectra of Si or Be δ-doped GaAs

Single Si or Be δ-doped layers in GaAs have been investigated by photoluminescence (PL) and photoluminescence excitation spectroscopy (PLE). Strong radiative recombination is observed from the two-dimensional electron or hole gas when the photocreated minority carriers are confined by GaAs/Al x Ga 1 − x As heterointerfaces placed at both sides of the doping spike. In n-type δ-doped structures the low-temperature PLE spectrum shows a well resolved Fermi edge enhancement with the energy position of the absorption edge independent of the excitation intensity. The spatially indirect band-to-band recombination, in contrast, is found to shift to higher energies with increasing excitation intensities due to screening of the space charge induced potential by photogenerated carriers. In the p-type δ-doped structure the low-temperature emission from the two-dimensional hole gas shows a pronounced enhancement in intensity at the Fermi edge.