Cr-doped Semi-insulating GaAs Research Articles

Characterization of semi-insulating GaAs:Cr by means of DC-CPM technique

In the present paper, constant photocurrent method, ‘CPM’ is used to determine the defects distribution from the optical absorption spectra of semi-insulating Cr-doped GaAs. By using the derivative method, we have extracted the distribution of the density of states DOS close to valence band edge Ev from the measured optical absorption spectrum. We have also developed a computer code program dc-CPM which have been used to compute the total absorption spectrum and their components using a model density of states inferred from two complementary techniques: constant photocurrent method, ‘CPM’ and transient photocurrent ‘TPC’. It is found that by combination of the two measurement techniques ‘CPM’ and ‘TPC’, and the two absorption spectra components and , we are able to have a full reconstruction of the density of states distribution below and above the Fermi level for such materials.

1/f2 spectra of decoherence noise on 75As nuclear spins in bulk GaAs

To identify the decoherence origin, frequency spectra using multiple π-pulses have been extensively studied. However, little has been discussed on how to define the spectral intensities from multiple-echo decays and how to incorporate the Hahn-echo T2 in the noise spectra. Here, we show that experiments based on two theories solve these issues. As proved in the previous theory, the spectral intensity is given as the decay in the long-time limit. Unlike the initial process of decays, this definition is not only theoretically proven but also validated experimentally, since long-time behaviors are generally free from experimental artifacts. The other is the fluctuation–dissipation theory, with which the Hahn-echo T2 is utilized as the zero-frequency limit of the noise spectrum and as an answer to the divergent issue on the 1/fn noises. As a result, arsenic nuclear spins are found to exhibit 1/f2 dependences over two orders of magnitude in all the substrates of un-doped, Cr-doped semi-insulating and Si-doped metallic GaAs at 297 K. The 1/f2 dependence indicates that the noise is dominated by a single source with characteristic frequency fcun = 170 ± 10 Hz, fcCr = 210 ± 10 Hz and fcSi = 460 ± 30 Hz. These fc values are explained by a model that the decoherence is caused by the fluctuations of next-nearest-neighboring nuclear spins.

Determination of defect level parameters in semi-insulating GaAs:Cr from transient photocurrent experiment

Transient photocurrent (TPC) measurements at several temperatures are used to determine the defect level parameters and the density of states distribution in semi-insulating Cr-doped GaAs. A two-dependent-step analysis is applied. Firstly, we determine the energy positions and the attempt-to-escape frequencies of the different defect levels from the Arrhenius plots of the corresponding emission times observed in the TPC decays. Secondly, we compute the density of states energy distribution g(E) from the same TPC decays using the Fourier transform technique with an exact matrix solution for g(E). The results obtained for this particular material are: four Gaussian bands, peaked around the energies 0.11, 0.21, 0.32 and 0.45 eV, with 1017, 1016, 5 × 1015 and 1015 cm−3 eV−1 as maximum values, respectively. This computed g(E) is introduced as a model of state distribution in an independent numerical simulation to reconstruct the experimental TPC data, and thus to validate the present analysis for g(E) determination. It is also found that the first TPC drop for times lower than 100 ns is dominated by the initial hole trapping. Full reconstruction requires then to take into account the hole current contribution in the TPC simulation at least at short times.

Mapping of GaAs wafers by IR light diffraction and luminescence

Optical investigations were carried out on undoped as well as on Cr-doped semi-insulating GaAs(100) wafers. The distribution of the non-equilibrium charge carrier grating erasure time τ E across GaAs wafers has been investigated by means of picosecond laser-induced dynamic grating and picosecond-photoconductivity techniques. Additional photoluminescence data have been obtained to reveal the contamination of GaAs wafers. The influence of high temperature annealing conditions on the distribution of the deep level EL2 and on the τ E mapping data has been studied. Also, feasibility of dynamic grating and picosecond-photoconductivity techniques for checking GaAs wafer quality is shown. Recombination through the deep levels EL2 and Cr are discussed.

Molecular beam epitaxy of In0.23Ga0.77Sb Grown on GaAs and GaSb substrates and the fabrication of planar In0.23Ga0.77Sb transferred electron devices

MBF In0.23Ga0.73Sb films are grown on semi-insulating Cr-doped GaAs and undoped GaSb substrates, and the first planar MBE-In0.23Ga0.77Sb transferred electron devices are fabricated and then evaluated. In the epitaxial growth process, an epitaxial layer structure such as the selection of a substrate and insertion of a high-resistivity buffer layer of Al0.9In0.1Sb is a significant factor influencing the electrical properties of the In0.23Ga0.77Sb active layer and then determine the threshold electric field of In0.23Ga0.77Sb/Al0.9In0.1Sb/GaSb epitaxial layer structure by the application of a high-resistivity buffer layer between the In0.23Ga0.77Sb film and the GaSb substrate. In the transferred electron device fabrication process, the contact properties of alloyed AuSn metallization to n-In0.23Ga0.77Sb film are also investigated and evaluated.

Improvement of implanted layer properties of semi-insulating GaAs by ingot annealing

The electrical properties and structural quality of wafers from lightly Cr-doped semi-insulating (SI) GaAs crystal before and after the three-step ingot annealing (IA) were studied. Comparison is based on the characteristics of seed and tail slices of the as-grown ingot and slices from seed and tail annealed slices cut normal to the growth axis. The carrier concentration profiles of implanted active layers were determined by C– V measurement with double mercury probe. Values of equivalent sheet carrier concentration, average low-field sheet Hall-effect mobility, and activation efficiency were obtained by measurement of the sheet Hall coefficient and the sheet resistivities of the square Hall-effect pattern. Spatially resolved cathodoluminescence (CL) of the investigated wafers is presented to support the interpretation of observed results.

Influence of the substrate on the photoconductivity of InGaAs/GaAs quantum wells

The in-plane photoconductivity of InGaAs/GaAs multiple quantum wells grown on Cr-doped semi-insulating GaAs substrates is found to display features originating from the substrate as well as from inter-sub-band associated transitions. Both the relative amplitude and the visibility of the spectral features are influenced by the substrate.

ZnSe field-effect transistors

We report the first demonstration of ZnSe metal-semiconductor field-effect transistors. These new devices were fabricated from n-type Cl-doped epitaxial ZnSe layers grown by molecular beam epitaxy on (100) oriented semi-insulating Cr-doped GaAs substrates. Epitaxial layers with room-temperature carrier concentrations of 1.6×1017 cm−3 and electron mobilities ranging from 400 to 500 cm2/V s were used for device fabrication. Au was used as a Schottky gate contact. Either In or a multilevel metallization scheme using Cr and In was employed for the source and drain ohmic contacts. Depletion-mode transistor operation was observed for structures with 5 and 100 μm gate lengths and varying gate widths. The 5 μm gate length by 200 μm gate width device structures exhibited transconductances of 0.5 mS/mm.

A new high-sensitivity characterization method of interface stress at heterostructures by Cr-related luminescence

A high-sensitivity characterization method for the interface stress at semiconductor heterostructures which uses the characteristic Cr-related luminescence lines in GaAs crystal is discussed. This method permits characterization of the interface stress with high sensitivity, reflecting the high sensitivity of the Cr-related luminescence lines to the strain field around the Cr luminescent center. This method has been applied to semiconductor heterostructures of InGaPAs/GaAs, AlGaAs/GaAs, and ZnSSe/GaAs which were fabricated on a Cr-doped semi-insulating GaAs substrate. >

Magnesium-doped In0.5Ga0.5P growth by liquid-phase epitaxy

The structural, electrical, and optical properties of Mg-doped In0.5Ga0.5P liquid-phase epitaxial layers, which were grown on (100) Cr-doped semi-insulating GaAs substrates with a supercooling method at 750 °C growth temperature, are reported. It has been found that the lattice mismatch between In0.5Ga0.5P epilayers and GaAs substrates are slightly increased with increasing Mg impurity concentration. The electrical properties of these epilayers were determined by Hall measurements in the temperature range between 77 and 300 K. The data show that both the room-temperature carrier concentration and the hole mobility of Mg-doped layers varied linearly from 3.5×1017 to 1.2×1019 cm−3 and from 50.5 to 15 cm2/V s, respectively, as a function of Mg dopant mole fraction from 1.0×10−4 to 1.5×10−3. The Mg acceptor ionization energy was evaluated as 21.3 meV. The photoluminescence results indicate that the Mg acceptor ionization energy in In0.5Ga0.5P is approximately 28–35 meV, which is different from the electrically determined value and is discussed in this report. The distribution coefficient is also determined in this report as a value of 0.12 for Mg in In0.5Ga0.5P liquid-phase epitaxy at 750 °C growth temperature.

Photorefractive two-wave mixing in GaAs using a diode-pumped Nd:YLF laser at 1.31 μm

Two-wave mixing has been observed in Cr-doped semi-insulating GaAs using a compact Nd:YLF diode-pumped solid-state CW laser system at 1.31 μm. Significant enhancement of the two-wave mixing gain was achieved through application of AC electric fields.

Vacancy complexes in Cr-doped GaAs

Cr-doped semi-insulating GaAs has been investigated by means of position lifetime spectroscopy. In as-grown GaAs the dominant positron trap is a Cr Ga·V As complex. Upon annealing the concentration of this complex increases around 260°C and then decreases at temperatures higher than 500°C. No vacancy agglomeration took place. In low temperature (130 K) e --irradiated (28 MeV) Cr-GaAs, di- and trivacancies were observed together with the gallium antisite, Ga As.

Studies of Photon Energy Dependence of Transient Photocurrent in Semi-Insulating GaAs Materials by means of Photo-Deep-Level Fourier Spectroseopy and Photosensitivity Transient Spectroscopy

The photon energy dependence of the transient photocurrent in Cr-doped semi-insulating GaAs materials was investigated by photo-deep-level Fourier spectroscopy (photo-DLFS) and photosensitivity transient spectroscopy (PSTS). From the experimental results, it was found that photo-DLFS signals change systematically according to the change in the photon energy of monochromatic light; fourteen deep levels were detected. It is shown that measurements taken while continuously varying the photon energy are very useful in studying specified deep levels in cases where the signals of two or more deep levels overlap and in detecting the many deep levels by increasing the change in the occupancy probability at each deep level. Also, a photoquenching effect is shown to occur within the new photon energy range, 0.85–0.92 eV, by PSTS.

Temperature and intensity dependence of photorefractive effect in GaAs

The photorefractive effect in semi-insulating Cr-doped GaAs as measured by the beam coupling technique was investigated as functions of temperature (295–386 K) and intensity (0.15–98 mW/cm2 of 1.15 μm light beams from a He-Ne laser). Results show that the photorefractive effect deteriorates rapidly over a narrow range of temperature as temperature rises and that this characteristic temperature increases with the logarithm of beam intensity. The observed phenomenon is attributed to the competing effects of the dark- and light-induced conductivities.

Rutherford backscattering measurements of dopant concentration in Cd-implanted gallium arsenide

Cadmium ions have been implanted in Cr-doped semi-insulating GaAs at an incident energy of 150 keV to a dose of 1 × 10 16/cm 2. The change in dopant concentration in GaAs after annealing has been examined by Rutherford backscattering techniques. The results obtained have clearly shown that a considerable amount of the implanted Cd atoms outdiffuse from the surface of the sample during annealing. The ratio of the total number of Cd atoms outdifussed to the implantation dose is more than eighty percent after annealing at 700°C for 2 min. It has also been revealed that the Cd loss due to annealing occurs even if annealing is done at low temperatures, e.g., at 200°C.

Redistribution of Cr in GaAs:Cr and of V in GaAs:V after implantation of Si, Be, or B and annealing in a controlled atmosphere

The redistribution of V in V-doped and Cr in Cr-doped semi-insulating GaAs substrates after implantation with Si, Be, B and subsequent controlled atmosphere annealing at 840 °C for 20 min is determined using secondary ion mass spectrometry. Both V and Cr show accumulation into zones of high defect densities at ∼Rp+ΔRp of the Si, Be, or B implantation profile, respectively, and a depletion which extends beyond that zone to ∼2–4×Rp. Some surface accumulation has also been observed. For all three cases studied, the V redistribution is much smaller than the Cr redistribution. It is therefore concluded that from thermal stability point of view semi-insulating GaAs:V should be a better substrate material for implantation type devices than is GaAs:Cr.

Photovoltaic study of the substrate-layer interface for Sn-doped GaAs grown by metalorganic vapor phase epitaxy

Photovoltaic spectra have been obtained for Sn-doped GaAs epitaxial layers grown by metal-organic vapor phase epitaxy (MOVPE) on Cr-doped semi-insulating GaAs substrates with a view to studying the substrate-layer interface characteristics. The photoresponse was found to be relatively strong and extended into the near infrared to λ≂2 μm. The photosignal is due to carrier excitation at the interface, where a high density of deep traps creates a depletion region and a potential barrier. The shape of the spectral response can be analyzed to yield information on the nature of these traps. They are found to be EL2 and Cr-related electron and hole traps in concentrations which depend on the layer growth conditions. The barrier height deduced directly from the spectra is about 0.6 eV, and is attributed to Fermi level pinning at the Cr2+ electron trap level.

Temperature dependence of FET properties for Cr-doped and LEC semi-insulating GaAs substrates

The temperature dependence of carrier concentration and mobility profiles is measured on Schottky-barrier gate FET's formed by Si implantation into intentionally Cr-doped Czochralski-grown semi-insulating GaAs substrates and LEC semi-insulating GaAs substrates. The effective depth of the channel implant is at least 100 nm shallower for Cr-doped substrates than it is for LEC substrates. Carrier mobilities are higher for LEC substrates; at low temperature, mobility is higher at the channel-substrate interface than it is nearer the surface. Devices on both types of substrates show free-carrier trapping effects between 160 and 225 K. For LEC substrates, traps are on the channel side of the active channel-substrate interface. For Cr-doped substrates, observation of traps depends on biasing the FET beyond pinchoff; traps are on the substrate side of the channel-substrate interface.

Surface morphology of Si(100), GaAs(100) and InP(100) following O 2+ and Cs + ion bombardment

Low energy ion bombardment is an important technique in the processing and analysis of electronic materials. Inert gas and reactive ions are used as the primary excitation agent in Secondary Ion Mass Spectrometry (SIMS) to obtain the depth distribution of dopants and contaminants in semiconductors. Ion etching is also used for pattern delineation in the fabriatiion of devices in the electronics and telecommunications industries. Although the technique is becoming increasingly important, ion erosion of surfaces does not always proceed uniformly and artefacts such as pits, pyramids, cones, facets and other ion-induced topographies are frequently observed. In this paper a systematic study of the effects of 5.5 keV ion bombardment on clean well-characterized surfaces of some important electronic materials (Si(100), Cr-doped semi-insulating GaAs(100) and Fe doped InP(100)) is reported using O 2 2 and Cs + primary ion species. Relatively high doses of10 19 ions cm −2 and 10 20 ions cm −2 have been used to ensure that the observed features are properly developed. Detailed studies of the morphologies have been made using scanning electron microscopy. The implications of the work for SIMS and the integrity of concentration-depth profiles are discussed.

Defect and photoluminescence profiles from thermally converted Cr-doped GaAs

Specimens of Cr-doped semi-insulating (SI) GaAs have been annealed in quartz ampoules under vacuum at elevated temperatures. Some samples, depending on temperature and time of anneal, were partially or wholly converted to p-type. In these cases CV measurements have been combined with serial sectioning to produce carrier concentration profiles. The As overpressure dependencies indicate acceptors to be associated with Ga vacancies. The diffusion coefficient of the Ga vacancies was estimated to be about 3.35 × 10 -14cm 2sec -1 at 950°C. Low temperature photoluminescence on the converted samples show a reasonably good correspondence between carrier distribution profile and the intensity of copper luminescence peak on photoluminescence spectra taken at various depth in the crystals.