SiGa Donors Research Articles

Self-trapped hole and impurity-related broad luminescence in β-Ga2O3

This work explores the luminescence properties of self-trapped holes and impurity-related acceptors using one-dimensional configuration coordinate diagrams derived from hybrid functional calculations. The photoluminescence spectrum of as-grown β-Ga2O3 typically consists of a broad band in the wavelength region from ultraviolet to green and is often dominated by an impurity independent ultraviolet band that is commonly attributed to self-trapped holes. Here, we use the self-trapped hole as a benchmark to evaluate the accuracy of the theoretical defect luminescence spectra and estimate the optical properties of MgGa, BeGa, CaGa, CdGa, ZnGa, LiGa, and NO acceptor impurities, as well as their complexes with hydrogen donors. We also explore VGa acceptors complexed with hydrogen and SiGa donor impurities. The results show that these defects can give rise to broad luminescence bands peaking in the infrared to visible part of the spectrum, making them potential candidates for the defect origin of broad luminescence bands in β-Ga2O3.

Modeling and interpretation of UV and blue luminescence intensity in β-Ga2O3 by silicon and nitrogen doping

Temperature-dependent cathodoluminescence spectra of (2¯01) Si-doped β-Ga2O3 single crystals and (010) N-doped epitaxial films were comprehensively shown to investigate their electronic structure and defect states. The decrease in the self-trapped exciton (STE) emission at low temperatures in heavily Si-doped crystals implied the Debye-Hückel screening of STEs with the critical charge density larger than 2 × 1018 cm−3. The analysis based on the rate equation model suggested a significant influence of the donor-acceptor-pair recombination involving a SiGa donor on the UV luminescence band in the Si-doped crystals. The blue luminescence band was suppressed for the heavily N-doped epitaxial films with a N concentration of 1 × 1018 cm−3, implying the decrease in the oxygen vacancy (VO) concentration by N doping. The increase in the NO acceptor concentration as well as the decrease in the VO concentration were found to contribute to the compensation of residual Si donors resulting in high resistivity of N-doped epitaxial films.

Band structure related wave-function symmetry of amphoteric Si dopants in GaAs

Autocompensated Si-doped GaAs is studied with cross-sectional scanning tunnelling spectroscopy (X-STS). The local electronic contrasts of substitutional Si(Ga) donors and Si(As) acceptors under the (110) cleavage plane are imaged with high resolution. Si(Ga) donor atoms exhibit radially symmetrical contrasts. Si(As) acceptors have anisotropic features. The anisotropic acceptor contrasts are traced back to a tunnel process at the valence band edge. They reflect the probability density distribution of the localized acceptor hole state.

Hydrogen passivation of the SiGa donor in GaP

AbstractIn LEC‐grown GaP doped with silicon, two vibrational absorption lines are measured at 2175.1 and 2190.3 cm–1 (T = 7 K). These lines are due to H stretching modes where hydrogen passivates SiGa donors. Uniaxial stress experiments show that the SiGa complex responsible for the 2175.1 cm–1 line has trigonal symmetry and is the isolated passivated SiGa. The complex creating the 2190.3 cm–1 line has the symmetry Cs and contains additionally a BGa with the four constituents located in an {110} mirror plane. Therefore, also the structure of the group‐IV donohydrogen complexes is in GaP different from that observed in GaAs.

Influence of the Substrate Orientation on the Silicon Capture into A- and B- Sublattices of Gallium Arsenide in Molecular Beam Epitaxy

The influence of crystallographic orientation of the growth surface near (100) and (111)A GaAs singular faces on the silicon capture into A- and B-sublattices of gallium arsenide in molecular beam epitaxy is investigated by the electrophysical and photoluminescence methods. It is demonstrated that the silicon dopand is incorporated into GaAs layers not only as simple SiGa donors but also as elemental SiGa acceptors and more complex defects, namely, SiAs–VAs complexes. The concentration of defects of different types in layers depends on the orientation of the growth surface, and the amphoteric properties of silicon on the (111)A face are manifested stronger than those on the (100) face.

Residual donors and compensation in metalorganic chemical vapor deposition as-grown n-GaN

In our recent report, [Xu et al., Appl. Phys. Lett. 76, 152 (2000)], profile distributions of five elements in the GaN/sapphire system have been obtained using secondary ion-mass spectroscopy. The results suggested that a thin degenerate n+ layer at the interface is the main source of the n-type conductivity for the whole film. The further studies in this article show that this n+ conductivity is not only from the contribution of nitride-site oxygen (ON), but also from the gallium-site silicon (SiGa) donors, with activation energies 2 meV (for ON) and 42 meV (for SiGa), respectively. On the other hand, Al incorporated on the Ga sublattice reduces the concentration of compensating Ga-vacancy acceptors. The two-donor two-layer conduction, including Hall carrier concentration and mobility, has been modeled by separating the GaN film into a thin interface layer and a main bulk layer of the GaN film. The bulk layer conductivity is to be found mainly from a near-surface thin layer and is temperature dependent. SiGa and ON should also be shallow donors and VGa–O or VGa–Al should be compensation sites in the bulk layer. The best fits for the Hall mobility and the Hall concentration in the bulk layer were obtained by taking the acceptor concentration NA=1.8×1017 cm−3, the second donor concentration ND2=1.0×1018 cm−3, and the compensation ratio C=NA/ND1=0.6, which is consistent with Rode’s theory. Saturation of carriers and the low value of carrier mobility at low temperature can also be well explained.

Atomic-scale properties of the amphoteric dopant Si in GaAs(110) surfaces

The electronic properties of positively charged Si Ga donors and negatively charged Si As acceptors in different subsurface layers as well as Si donor–Si acceptor dipoles and uncharged Si pairs in GaAs(110) surfaces were investigated by scanning tunneling microscopy. The subsurface location of the dopant atoms is determined from the observed symmetry. Localized states of the dopant atoms are identified. The fraction of Si atoms incorporated in GaAs on As sites increases with increasing Si concentration. Si pairs are found to be incorporated substitutionally in GaAs and their contrast indicates the presence of half-filled dangling bonds. The Si Ga donor–Si As acceptor dipoles are surrounded by a height change indicating a dipole screening field.

Lattice Sites of Silicon Impurities in AlGaAs Grown by Liquid Phase Epitaxy

Localised vibrational mode infrared absorption (10 K) and Hall measurements were made on a series of Si doped AlxGa1-xAs samples with 0 < x < 0.25 grown by liquid phase epitaxy. Localised vibrational modes were detected from SiGa donors, Sim acceptors and SiGa—SiAs pairs which increased in frequency as x increased. The assignments of new lines observed at 386, 388 and 391 cm -1 are discussed in relation to possible perturbations of the lines from SiG a or SiAs. The presence of DX centres was inferred from observed persistent photoconductivity and attempts were made to relate this result to the presence of the new IR lines.

Si delta -doping in GaAs: investigation of the degree of confinement and the effects of post-growth annealing

A stack of 60 delta -planes, each containing a Si areal concentration of 3.4*1014 cm-2 ( approximately 0.5 monolayers, ML), grown in GaAs by molecular beam epitaxy at 400 degrees C has been examined before and after post-growth annealing by high-resolution X-ray diffractometry, transmission electron microscopy, secondary-ion mass spectrometry and infrared absorption localized vibrational mode (LVM) spectroscopy. These techniques provided complementary information concerning the concentration, spatial distribution and site occupancy of the Si atoms. It was found for the as-grown samples that the Si was located on Ga lattice sites (SiGa) and confined to layers no more than 2 ML in thickness. Annealing at 600 degrees C resulted in spreading of the delta -layers, with some Si remaining on the original planes, and the remainder diffusing away, resulting in a Si concentration of 2.1*1019 cm-3 between the delta -planes. LVM spectroscopy indicated that the diffused Si atoms were present as SiGa donors, SiAs acceptors, SiGa-SiAs pairs and Si-X complexes. After the samples were annealed at 800 degrees C or 950 degrees C only a uniform Si concentration of 3*1019 cm-3 was detected, although there was excess Si in the surface region where dislocation loops were observed. It is concluded that there is a maximum silicon concentration in solution that is in equilibrium with the delta -layers at 600 degrees C and that the Si in the delta -layers is present as dimers or larger 2D clusters.

A local vibrational mode investigation of p-type Si-doped GaAs

Infrared absorption (IR) and Raman scattering measurements have been made of the localized vibration modes (LVM) due to defects incorporating silicon impurities in p-type Si-doped GaAs grown by liquid phase epitaxy (LPE) on (001) planes and by molecular beam epitaxy (MBE) on (111)A and (311)A planes. Analysis of a closely compensated LPE sample indicated that an existing calibration factor for the SiAs LVM (399 cm−1) relating the integrated absorption coefficient (IA) to the concentration [SiAs] should be increased by 40%, so that IA=1 cm−2 corresponds to [SiAs]=7×1016 cm−3. The SiAs LVM appeared as a Fano dip in the hole absorption continuum at ∼395 cm−1 in the highly doped p-type material, some 4 cm−1 lower in frequency than its normal position in compensated GaAs. Electron irradiation of samples led to the progressive removal of the Fano dip and a shift with the emergence of the expected SiAs LVM absorption line at 399 cm−1. In MBE samples the irradiation also generated SiGa donors, but the site switching was not detected in LPE material. By contrast, Raman spectra of as-grown p-type samples exhibited a symmetrical peak at 395 cm−1, which also shifted towards 399 cm−1 as the free carriers were removed. MBE (111)A GaAs:Si compensated by SnGa donors revealed the SiAs LVM at its normal position. After hydrogenation of MBE and LPE samples, only stretch modes due to H-SiAs were observed. Passivated MBE GaAs (111)A codoped with Si and Be showed stretch modes due to both shallow acceptors. It was thereby concluded that only one type of acceptor (SiAs) was present in p-type Si-doped GaAs, contrary to previous proposals. There was no evidence for the presence of SiAs pairs or larger clusters.

Si donors (SiGa) in GaAs observed by scanning tunneling microscopy

We report the direct observation of SiGa donors in the top six layers of GaAs(110). Surface SiGa has a localized electronic feature due to its dangling bond, while subsurface SiGa produces delocalized protrusions in filled and empty state scanning tunneling microscopy images, with a full width at half‐maximum of ∼25 A and a height from ∼0.2 to 2 A, depending on the sample bias and location under the surface. The delocalized features of subsurface SiGa can be understood in terms of the perturbation of the local band structure by the Coulomb potential of SiGa.

Silicon donor-hydrogen complex in GaAs: A deep donor?

Post-hydrogenation anneals of shallow SiGa donors in GaAs indicate that their reactivation rate is enhanced in the presence of an applied electric field. We show that existing data are consistent with the SiGa-H complex being a deep donor dissociating only via its ionized state. The 0/+ level of this deep donor is found to be at EC−0.75 eV. There is no need to appeal to the existence of negatively charged hydrogen to account for the reactivation of SiGa donors.

The vibrational modes of silicon acceptors in p-type GaAs grown by molecular beam epitaxy on a (111)A plane

Silicon impurities in p-type GaAs grown by molecular beam epitaxy occupy As lattice sites and give rise to an infrared active localized vibrational mode (LVM) which appears as a Fano profile near 395 cm−1 in samples with p=1019 cm−3 or 1020 cm−3. 2 MeV electron irradiation reduces p, leading to the emergence of the usual LVM absorption line of SiAs at 399 cm−1. The treatment also leads to site switching to produce SiGa donors giving an LVM at 384 cm−1.

The lattice locations of silicon atoms in delta-doped layers in GaAs

We have used secondary ion mass spectrometry, local vibrational mode infrared absorption, and electrical characterization to study the incorporation of Si delta-doped planes in GaAs grown by molecular beam epitaxy at 400 °C, in the concentration range 0.01–0.5 monolayers. A correspondence is observed between the density of SiGa donors, the free electron concentration and the total Si coverage, up to a coverage of ∼1013 cm−2; however, above this value, the electron density falls, while [SiGa] remains constant up to a coverage of ∼1014 cm−2, and then falls below the detection limit at 0.5 monolayer coverage. These effects have been interpreted in terms of a model which takes account of Si migration and aggregation on the delta-doped plane during deposition.

Thermal stability of dopant-hydrogen pairs in GaAs

The thermal stability of dopant-hydrogen complexes in hydrogenated n- and p-type GaAs(1–2×1017 cm−3) has been determined by examining their reactivation kinetics in reverse-biased Schottky diodes. The reactivation process is first-order for all of the dopants, with thermal dissociation energies (ED) of 1.45±0.10 eV for SiAs acceptors, 1.25±0.05 eV for SiGa donors, 1.20±0.10 eV for SnGa donors, 1.25±0.10 eV for Zn acceptors, 1.35±0.05 eV for CAs acceptors, and 1.15±0.10 eV for Be acceptors. The dissociation frequencies (ν) are thermally activated of the form νD = ν0E−ED/kT, with the ν0 values in the range 1–5×1013 s−1. The results are consistent with much of the H being present as H+ in p-type material, and H− in n-type material.

Type conversion of epitaxial GaAs layers after heavy ion MeV implantation and annealing

Heavy ion (erbium, 167Er) MeV implantation and annealing of thick epitaxial layers of n-type GaAs has been found to result in the conversion of a surface layer within the erbium profile to p-type. Similar effects in originally p-type and undoped, high resistivity layers are not observed. In addition, no active role is ascribed to the erbium in providing mobile holes. The source of the type conversion is attributed to the transfer of Si Ga donors to Si As acceptor sites as the result of the implantation and annealing.

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. Possible structures for Si-X are discussed but a definitive model cannot yet be proposed. MBE samples grown at 400 °C had values of [n] close to 1019 cm−3, and a negligible concentration of Si-X. On annealing, [n] decreased and Si-X defects were produced together with site switching of Si(Ga) to Si(As). These results are important to the understanding of the mechanism of silicon diffusion at low temperatures. The infrared absorption and Raman measurements are complementary. Absorption measurements made at a resolution of 0.1 cm−1 require layers greater than or equal to 1 μm in thickness doped to a level of 3×1017 cm−3 but require the prior elimination of free-carrier absorption. Raman measurements can be made on as-grown layers only 10 nm in thickness doped to a level of 2×1018 cm−3, but with a spectral resolution of only 5 cm−1.

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.

Si-defect concentrations in heavily Si-DOPED GaAs: annealing-Induced changes-II

Isothermal annealing produces changes in the free carrier density, defect-induced localized vibrational mode (LVM) infrared absorption, microstructure as measured by transmission electron microscopy (TEM), and critical resolve shear stress of heavily Si-doped GaAs. The changes have been measured and correlated for three different Si concentrations for several annealing temperatures. The measurements reveal temperature dependent annealing-induced changes in several specific defect concentrations. The observations indicate the following behavior for two ingots with [Si] ≳ 2 × 1019 cm−13: (1) when the anneal temperature, TA = 400°C, the concentration of Siga donors, as determined from LVM spectra decreases probably due to the generation of VGa defects followed by the formation of SiGa-VGa pairs. This change is responsible for observed decreases in carrier density and the large increase in yield stress. The yield stress shows a dependence of the form σ-σo ∝ [SiGa-VGa]1/4. (2) When TA = 500°C, the LVM spectra indicate that all of the observed Si defect concentrations change. The decrease in [SiGa] alone cannot explain the decrease in carrier density, and a previous suggestion that a new acceptor is required is confirmed. Both the LVM measurements and the shear stress indicate that only a small fraction of the [SiGa] reduction is by the formation of SiGa-VGa pairs. (3) When TA = 700°C, a new acceptor is still required and the other experimental observations at TA = 500°C are also still seen here. There is a large decrease in [SiGa] and [SiAs] observed for short anneal times which coincides with the formation of Si-rich extrinsic loops and the loop area/vol increases with [Si]. (4) When TA > 700°C, all of the changes become smaller as TA increases. For lower [Si] ~ 1.5 × 1018 cm−3, no significant annealing-induced changes are observed for any of the TA given above.

The complex form of donor energy levels in gallium phosphide

The conduction band minima of GaP are now recognised to be exceptionally non-parabolic because of the existence of a 'camel's back' form. This has a most serious effect on donor bound states of binding energy 10-20 meV, just the range of the p-states available for a fit by effective mass theory (EMT) for spheroidal conduction band extrema. In the absence of a detailed EMT for such a complicated band structure the authors argue the approximate applicability of a variable gamma model. This model is applied to the present knowledge of donor excited states obtained from electronic Raman scattering, bound exciton 'two-electron' luminescence satellites and direct photoexcitation spectra. The parentage of the excited states observed in these different experimental techniques is critically reviewed. It is shown that the variable gamma model has some merit; in particular gamma is seen to increase with the binding energy of the donor state described especially for the deep 1s state, the expected trend. The first Zeeman data is presented for the donors SiGa, SP and TeP. Several of the SiGa donor p-excited states split as expected although a weak transition to a state unanticipated on the EMT splits lika a p+or- state and magnetic splittings of the p+or- levels are unexpectedly state-dependent.