B. R. Davidson

Carbon acceptors passivated with hydrogen and the search for carbon donors in highly doped GaAs:C

Highly carbon doped p-type GaAs grown by MOVPE shows infrared absorption lines from localized vibrational modes of CAs acceptors and passivated H-CAs pairs. A comparison with samples grown by MOMBE shows that an unassigned line at 563 cm-1 is due to a mode of H-CAs pairs, although an isotopic analogue from D-CAs pairs in plasma-treated samples has not been detected. The important new result is the demonstration that the line cannot be due to CGa donors, and so there is still no evidence for the amphoteric behaviour of carbon impurities in GaAs.

A calibration of the H-CAS stretch mode in GaAs

GaAs samples grown by chemical beam epitaxy and doped using 12CBr4 or 13CBr4 to give carbon concentrations of ~ 1× 1019 cm-3 have been analysed by local vibrational mode infrared spectroscopy, secondary-ion mass spectrometry, Hall measurements and electrochemical capacitance - voltage profiling. By exposure to a hydrogen plasma ~ 80% passivation of the carbon was achieved. A calibration for the stretch mode of H-CAS pairs in GaAs has been established indicating that an integrated absorption of 1cm-2 in the LVM line at 2635.2cm-1 (H-12CAS) corresponds to a concentration of 9 ± 2 × 1015 pairs/cm3. This calibration is approximately half that of the most recently published value. Reasons for the difference are discussed.

The lattice sites of carbon in highly doped AlAs:C grown by molecular beam epitaxy

Infrared absorption measurements of heavily carbon-doped AlAs have revealed a localized vibrational model at 630 cm-1 which appears as a Fano profile. Raman scattering measurements show a corresponding line at 634 cm-1: possible reasons for the difference in frequency are discussed. The line is assigned to CAs acceptors and a comparison is made with a two-parameter Keating cluster model. Additional weaker lines at 615 cm-1, 633 cm-1 and 645 cm-1 have been observed in absorption, and possible interpretations have been considered.

Di-carbon complexes in AlAs and GaAs

Heat treatment of heavily carbon doped AlAs and GaAs results in a loss of CAs shallow acceptors. In Raman scattering experiments on annealed CBE grown GaAs with 12C and 13C isotopes, and MOVPE grown AlAs it is found that the loss of carriers is accompanied by the appearance of two high frequency lines. These lie near to the stretch mode of an isolated C2 molecule (1855 cm−1). This is consistent with the formation of two types of di-carbon defects in these materials where the C atoms are bonded together and one or both of which act as a donor. Using a local density functional method to investigate the structure and dynamics of several di-carbon defects, we find that the dimer at an As site is bistable and aligned approximately in a [100] direction in the neutral charge state, and in a [110] direction when positively ionised. The calculated frequencies lie within 10% of the measured values in both materials. Other defects are investigated too with a view of determining the structures giving rise to the modes.

Characterization of carbon delta-doping GaAs superlattices grown by chemical beam epitaxy using CBr4

Abstract Carbon delta-doping GaAs superlattices incorporating 50 layers have been grown by chemical beam epitaxy (CBE) with or without interrupting the triethylgallium (TEGa) flux during exposure of the surface to CBr 4 to effect the doping. Infrared localized vibrational mode (LVM) spectroscopy showed that the carbon was incorporated on the As sublattice and high-resolution X-ray diffractometry (HRXD) was used to measure the strain in the layers. For the sample doped during TEGa flux interruption more than 6 orders of satellite peaks were observed on either side of the average layer peak. Simulation of the HRXD profile using dynamical diffraction theory showed that the δ-doped layers were about 5 A thick with a carbon concentration of 2.9 × 10 20 cm −3 . Electrochemical capacitance-voltage profiling of this structure gave a peak concentration of 2 × 10 19 cm −13 with a width of 27 A, which corresponds to the limit of spatial resolution for this technique.

Hydrogen wag modes and transverse carbon modes of H-CAs complexes in GaAs doped with 12C or 13C

Infrared absorption measurements have been made of the vibrational modes of H-12CAs, D-12CAs, H-13CAs and D-13CAs pairs in GaAs. All modes of A1-symmetry have been identified, together with the two E-modes of D-12CAs and D-13CAs pairs and the transverse carbon E-modes of paired 12CAs and 13CAs impurities. The D-wag modes lie below the transverse carbon E-modes, while the H-wag modes lie at higher frequencies than the carbon modes. An anticrossing behaviour explains why the transverse modes of D-CAs pairs occur at higher frequencies than those of H-CAs pairs.

Carbon delta doping in chemical beam epitaxy using CBr4

Abstract We describe the growth of carbon δ-doped GaAs by chemical beam epitaxy (CBE) using CBr 4 as the dopant during growth interrupts. Characterisation of the δ-doped samples using secondary ion mass spectrometry (SIMS) at a series of impact energies showed that the C was confined to a planar sheet less than 1 nm thick. This is in agreement with HRXRD measurements on C δ-doping superlattices. The results of SIMS profiling of layers with a range of interrupt times indicate that there is an initial surface coverage of C from CBr 4 of approximately 9 × 10 12 cm −2 which increases relatively slowly during an extended interrupt.

Reflectance anisotropy spectroscopy studies of the growth of carbon-doped GaAs by chemical beam epitaxy

Reflectance anisotropy spectroscopy (RAS) was used to study the carbon doping of GaAs grown by chemical beam epitaxy. Intrinsic carbon doping using trimethylgallium (TMGa) and arsine at a substrate temperature of 480°C was compared with extrinsic carbon doping using CBr4, triethylgallium and arsine. Samples were grown under a range of V:III ratios and the resultant surface reconstructions were determined by RAS. For samples grown under a (2 x 4)-like reconstruction the RAS spectra taken immediately after growth showed a strong (2 x 4)-like character rather than a return to a c(4 x 4)-like reconstruction as is seen for undoped GaAs. This was seen for both intrinsic and extrinsic doping and is attributed to a shift in the As-dimer bond energy for highly doped GaAs : C. Features in the RAS spectra at 3.1 eV which developed during the cooling of the samples and which were clearly related to the carbon doping level were attributed to the linear electro-optic effect caused by doping of the underlying layer and not to surface structural effects. The C-doping level obtained using TMGa is known to vary strongly with V : III ratio and this was confirmed by post-growth analysis. IR spectroscopy of heavily doped samples grown using TMGa showed the presence of an aligned carbon complex (H-(CAs) 2 ), lying parallel to [1 1 0] which has previously been seen in GaAs grown by metal-organic molecular beam epitaxy (MOMBE).

Carbon δ-doping GaAs superlattices

Carbon δ-doping GaAs superlattices incorporating 50 layers have been grown either by chemical beam epitaxy (CBE) using CBr4 as the source of the carbon or by metallorganic vapour phase epitaxy (MOVPE) using CCl4. Infrared (IR) localized vibrational mode (LVM) spectroscopy showed that carbon atoms were incorporated on the As sublattice and that hydrogen was incorporated as H-CAs pairs during growth by MOVPE. The hydrogen was removed by an anneal at 600 ‡C for 15 min in N2, leading to an increase in the carrier concentration. High resolution X-ray diffractometry (HRXD) was used to measure the strain in the layers. Simulation of the 400 diffraction profiles using dynamical theory was used to determine the thickness (~ 1 nm) and peak concentrations (in excess of 2 × 1020 cm−3) of the δ-doped layers. Device applications and δ-layers in AIAs are briefly considered.

The suppression of misfit dislocation introduction in heavily carbon doped GaAs

The relaxation of heavily carbon doped GaAs, grown on (001) GaAs substrates was investigated using transmission electron microscopy and x-ray diffraction. Misfit dislocation introduction occurred only significantly above Matthews and Blakeslee’s critical thickness, with the onset of relaxation being delayed as the C concentration was increased. The resultant strain relaxation was highly anisotropic with the introduction of As-cored misfit dislocations being totally suppressed at the highest C levels, resulting in cracking of the epilayer. Locking of As-cored partial dislocations and a reduction in misfit dislocation velocity is proposed as the cause of the anisotropy and lack of relaxation.