J. J. Harris

The calibration of the strength of the localized vibrational modes of silicon impurities in epitaxial GaAs revealed by infrared absorption and Raman scattering

n‐type silicon‐doped epitaxial layers of gallium arsenide grown by molecular‐beam epitaxy (MBE) or metal‐organo chemical vapor deposition (MOCVD) have been investigated by measurements of the Hall effect and the strengths of the localized vibrational modes (LVM) of silicon impurities using both Fourier transform absorption spectroscopy and Raman scattering at an excitation energy of 3 eV close to the E1 band gap. Lines from Si(Ga) donors, Si(As) acceptors, Si(Ga)‐Si(As) pairs, and Si‐X, a complex of silicon with a native defect, were detected and correlated for the two techniques. The maximum carrier concentration [n] found for samples grown under standard conditions was 5.5×1018 cm−3. At higher doping levels Si‐X becomes dominant and acts as an acceptor, so reducing [n]. An integrated absorption of 1 cm−2 in the Si(Ga) LVM line corresponds to 5.0±4×1016 atoms cm−3: a similar calibration applies to the Si(As) line, but for Si‐X, an absorption of 1 cm−2 corresponds to only 2.7±1.0×1016 defects cm−3. Possib...

Mechanism of compensation in heavily silicon‐doped gallium arsenide grown by molecular beam epitaxy

Silicon‐doped GaAs grown by molecular beam epitaxy has been characterized by Hall measurements, infrared local vibrational mode (LVM) absorption, secondary ion and laser source mass spectroscopy. Highly doped samples with [Si]∼3×1019 cm−3 show only a low carrier concentration of 8×1017 cm−3. LVM spectroscopy shows that SiGa donors are compensated predominantly by [Si‐X] complexes, where X has been assigned previously to a gallium vacancy (VGa). Other compensating impurities are not present in the layers at significant concentrations.

Migration of Si in δ-doped GaAs

Si atomic plane or delta ( delta )-doping of GaAs during MBE has been investigated using SIMS profiling optimised for high depth resolution. For layers in which almost all the Si atoms act as donors, post-growth diffusion occurs at the growth temperature with a diffusion coefficient estimated to be 9*10-17 cm2 s-1 at a substrate temperature of 550 degrees C. At temperatures greater than about 550 degrees C, a marked preferential migration towards the surface is observed, which may be due to surface segregation or possibly enhanced diffusion.

Delta-doping of GaAs and Al0.33Ga0.67As with Sn, Si and Be: a comparative study

Abstract We have performed a study of the contributions of segregation, diffusion and aggregation to the broadening of delta-doped planes of Sn, Si and Be in GaAs and Al 0.33 Ga 0.67 As. Sn planes are severely broadened by all three processes, but sharp spikes of Si and Be can be obtained in both host materials for sheet densities below 10 13 cm −2 and growth temperatures of 500°C or less. At higher temperatures or densities, segregation or concentration-dependent rapid diffusion may occur, causing significant spreading even during growth. Co-deposition of Si and Be dramatically reduces this broadening, and various mechanisms are considered to explain these effects.

Electronic transport in low-dimensional structures

Reviews the transport properties of electrons in semiconductor heterojunction structures, in which the degrees of freedom for motion of the charge carriers are reduced by confining potentials, thereby producing low- (i.e. two-, one- or even zero-) dimensional electronic structures. A theoretical treatment of quantisation effects due to two-dimensional (2D) and one-dimensional (1D) confinement, with and without an applied magnetic field, will be given, and used to interpret a range of experimental results obtained from GaAs/AlxGa1-xAs 2D and 1D heterojunction structures. For 2D samples, these results cover low-field mobility measurements, which give information on the scattering mechanisms present, and high-field magnetotransport studies, including the quantum Hall effect and parallel conduction processes. In 1D structures, quantum and ballistic transport processes will be discussed.

Post-growth diffusion of Si in delta -doped GaAs grown by MBE

A GaAs layer grown by MBE at a substrate temperature of 520 degrees C and containing three delta -doped planes with Si concentrations of 0.4, 1 and 4*1013 atoms/cm2 has been post-growth annealed in a furnace, up to temperatures of 648 degrees C. Secondary ion mass spectroscopy (SIMS) and capacitance voltage (cv) measurements have been carried out to measure profile-broadening. The most lightly doped plane gave a near-Gaussian diffusion profile with a diffusion coefficient comparable with literature values for simple diffusion of isolated SiGa atoms. The more heavily doped planes exhibit a complex profile shape with two components, a proportion of the atoms being confined to the original plane, together with an almost square-shaped profile of fast-diffusing atoms. Comparison of the CV and SIMS data suggests that formation of Si islands is taking place during deposition of the delta -doped plane, giving electrically inactive atoms which can subsequently diffuse into the surrounding GaAs during heat-treatment. This model is supported by preliminary local vibrational mode measurements which have been made on a set of multiplane samples.

Two-dimensional electron gas structures with mobilities in excess of 3×106 cm2 V−1 s−1

We have demonstrated that the mobility of two‐dimensional electron gas (2DEG) structures shows a gradual improvement with the number of uninterrupted growth runs in the molecular‐beam‐epitaxy system. Some, but not all, of this improvement can be attributed to the cleanup of the GaAs layers in the structure, and we suggest that there is a corresponding cleanup of the (Al,Ga)As which is also influencing the mobility. Once the system has passed the cleanup phase, a systematic trend of carrier density and mobility with undoped spacer thickness was observed, with a peak mobility at 4 K of 2.12×106 cm2 V−1 s−1 for a sheet carrier density of 2×10−11 cm−2 occurring at a spacer thickness of 800 A. A further increase in mobility was achieved by using a thicker region of doped (Al,Ga)As, thereby moving the ionized centers in the surface depletion layer further away from the 2DEG. This has enabled us to produce a sample with mobility at 4 K of 3.1×106 cm2 V−1 s−1 (at a sheet charge of 3.1×1011 cm−2)—the first time su...

Spin configurations and quasiparticle fractional charge of fractional QHE ground states in the N=0 Landau level

Abstract Spin configurations of fractional states v= p q in the N=0 Landau level are examined by in-situ rotation studies of an exceptionally high quality sample. The 2 3 and 4 3 parent states are found to be unpolarised at low B, as is the 8 5 daughter state (of the polarised 5 3 parent), and field-induced phase transitions for these states are observed. The validity of the probe σ c xx =σ xx ( 1 T =0) ≏ ( e q ) 2 /h of the quasiparticle fractional charge e ∗ =± e q is confirmed by extensive integer QHE activation studies, from which a universal prefactor σcxx close to e 2 h is identified.

Quantitative assessment of Be acceptors in GaAs by local vibrational mode spectroscopy

Be‐doped epitaxial layers of GaAs grown by molecular beam epitaxy have been studied by local vibrational mode spectroscopy combining infrared absorption and Raman scattering. Calibration factors for both experimental techniques have been derived which enable quantitative assessments to be made of the concentrations of Be acceptors in GaAs. In Raman spectroscopy the detection limit is ≊ 3×1018 cm−3 for as‐grown layers only 10 nm in thickness.

Si migration effects in GaAs/(Al,Ga)As heterojunction and δ-doped structures

Abstract In order to investigate the Si migration mechanisms thought to be responsible for the anomalous characteristics of a number of heterojunction devices, high depth-resolution SIMS profiling has been used to study these effects in δ-doped and uniformly-doped GaAs samples. For growth at or above ≈ 500°C, surface segregation occurs during the deposition phase, followed by a post-growth diffusion process; the rate of diffusion differs in different Si concentration regimes.