C. Ghezzi

Electrical and photoluminescence properties of undoped GaSb prepared by molecular beam epitaxy and atomic layer molecular beam epitaxy

Abstract High-quality, unintentionally doped GaSb layers were grown by molecular beam epitaxy (MBE) and by atomic layer MBE (ALMBE) to study the influence of growth conditions on their transport and photoluminescence properties. While the hole mobility μ is only slightly dependent on the growth conditions, the 77 K hole concentration p shows minima at growth temperatures of ∼ 450°C and for Sb 4 /Ga beam equivalent pressure ratios (BEPRs) of ∼ 8; the concentrations of singly and of doubly ionizable acceptors and of a compensating donor have been obtained by simultaneously fitting the temperature dependence of μ and p in the 40–300 K range; from these results we show that the existence of minima of p can be related to an increased electrical compensation due to a reduced incorporation of acceptors. The 15 K photoluminescence spectra are generally dominated by the donor-acceptor pair transition (A, 779 meV) involving the neutral state of the native acceptor, except when the growth is carried out at relatively low temperatures with low BEPRs. Under the latter conditions: (i) a peak (788 meV) shows at an energy 25 meV below the band-gap energy and (ii) exciton related transitions have intensities comparable to that of the A recombination.

Electron mobility and physical magnetoresistance in n-type GaSb layers grown by molecular beam epitaxy

Electron mobility and low-field transverse physical magnetoresistance were measured in Te-doped GaSb layers grown by molecular beam epitaxy. The samples investigated had electron densities ranging from to ; measurements were taken in the 8 - 300 K temperature range. The high mobility values demonstrate that SnTe can be used as a source of Te doping with results comparable with GaTe. A detailed analysis of the magnetoresistance data demonstrates that in samples with high electron density the magnetoresistance is mainly due to mixed conduction of electrons in both and L conduction band minima: the analysis gives the temperature dependence of the and mobilities and of the energy separation between L and edges. is 82 meV at 300 K and 67 meV at 8 K and exhibits a non-monotonic behaviour within the temperature range explored. In samples with low electron density the magnetoresistance is mainly due to the energy distribution of carriers in the valley.

Preparation of GaSb by molecular beam epitaxy and electrical and photoluminescence characterization

Abstract The molecular beam epitaxy and the characterization of high-quality, non-intentionally doped and Te-doped GaSb are reported. The undoped layers have 77 K Hall hole concentrations p = 1−3 × 10 15 cm −3 and mobilities μ = 3000−5600 cm 2 V −1 s −1 , which compare favourably with the best results reported so far. Photoluminescence (PL) measurements at 70 K point to a relatively low concentration of intrinsic GaSb antisite defects, responsible for the p-type behaviour. A simultaneous fit of μ and p was performed in the 40–300 K temperature range by using a model with two acceptors, one of them having two charge states; the levels have energies consistent with those deduced by PL experiments. n-Doping was obtained approximately in the range 1 × 10 16 −1 × 10 18 cm −3 by using Te from an SnTe source. The mobility values are definitely higher than those reported in the literature and obtained with the same source, and they slightly exceed those achieved in GaSb doped using a GaTe Te source. A procedure for the analysis of electrical data has been set up and tested for n-GaSb with free carrier concentrations lower than a few 1017 cm−3.

Electrical characterization of self-assembled InAs/GaAs quantum dots by capacitance techniques

Abstract The electrical characteristics of GaAs/InAs/GaAs structures containing self-assembled quantum dots (QD) or pseudomorphic layers (PSL) of InAs have been investigated by capacitance–voltage ( C – V ) measurements and deep-level transient spectroscopy (DLTS). The depth profiles of the apparent electron concentration obtained by C – V measurements show significant carrier depletion centered around the position of the InAs layer on both QD and PSL samples. In contrast, an accumulation peak, whose position depends on the temperature and the test signal frequency, is detected at low temperature only on QD samples. In addition to the M1, M3, and M4 traps, which are commonly detected in GaAs grown by molecular beam epitaxy (MBE), DLTS investigations show two InAs-related levels located at 60 and 480 meV below the GaAs conduction band edge. The shallower level, which is observed only on QD samples, is associated with an energy level induced by the dots. The deeper level, detected on both QD and PSL samples, is due to defects related to the InAs insertion. The influence of the above levels on the C – V characteristics is discussed.

DLTS AND PHOTOLUMINESCENCE OF MBE GaAs GROWN IN THE PRESENCE OF HYDROGEN

Abstract We report here electrical transport, deep level transient spectroscopy (DLTS) and low-temperature photoluminescence (PL) studies aimed at investigating the effect of hydrogen on deep and shallow levels in Sn doped GaAs, prepared by solid source MBE, in the presence of a 1 10 -6 Torr hydrogen back-pressure. Electrical transport and PL data show that the use of hydrogen results in a significant decrease of the unintentional shallow donor and acceptor concentration. As a result, high electron mobility (84200 cm 2 /V·s at 77 K) and high PL quality GaAs could be prepared. The M1, M3, and M4 deep electron trap concentrations were found to decrease by one order of magnitude in layers grown at either 580 and 640°C, in the presence of hydrogen. The deep level concentration of samples grown at 640°C with hydrogen are in the low 10 11 cm -3 range. The effect of hydrogen may be due either to the incorporation of shallow and deep levels at a reduced rate or to their passivation. Different tentative explanations are suggested, which, to some extent, can be supported by literature dealing with post-growth hydrogenation treatments of crystalline GaAs and of clean GaAs surfaces.

Use of spatially dependent electron capture to profile deep‐level densities in Schottky barriers

A new procedure to profile deep‐level densities within the space‐charge region of Schottky barriers is presented. The method takes advantage of the spatial dependence of the time constant for the free‐electron capture by deep‐donor traps. The amplitude of the slow component of the capture capacitance transient following a negative reverse voltage pulse is simply related to the trap density at the point where the Fermi level crosses the trap level itself. The density profile of a given trap can be achieved by measuring the slow‐component transient signal as a function of the reverse voltage at a suitably chosen constant temperature. The estimated spatial resolution of the method was near 50 A in a practical case. Experimental density profiles for EL14, EL8, EL3, and the EC−ET =0.37 eV level in Cr/GaAs and Al/GaAs Schottky barriers are presented and discussed. The procedure is expected to be also applicable to the case of trap densities comparable with the shallow‐donor density without introducing large errors.

Epitaxial growth of single crystal Cd1-xZnxS layers on (111) GaAs substrates using the close-spaced geometry

Single crystal Cd1-xZnxS layers have been epitaxially grown on (111)GaAs substrates by a vapour phase chemical transport method using the close-spaced geometry and H2 as a transport agent. The usefulness of the technique for growing layers with a given controlled composition was systematically investigated up to a ZnS molar fraction of about 0.65. Results obtained with sources made by mechanical mixtures of CdS and ZnS powders and with solid solution sources were analyzed and discussed. Composition gradients in the direction of the c-hexagonal axis were observed in both cases but with different sign. Crack free layers with an uncertainty Δx=±0.015 in the composition near the film—substrate interface were obtained using mechanical mixture sources.

DX-CENTER-RELATED FEATURES BY CAPACITANCE MEASUREMENTS IN ALGAAS

The role of quasistatic C‐V measurements in investigating DX‐center‐related features in AlGaAs (x=0.25, 0.30, and 0.35) Schottky barriers has been reconsidered under different experimental conditions. The vanishing of the electron‐capture rate by the DX center, at low temperature, is responsible for a frozen‐step‐like density profile of positively charged DX centers near the metal‐semiconductor interface. This causes a knee‐shaped 1/C2‐vs‐V plot and gives rise to an apparent built‐in potential. The low‐temperature freezing in of the free‐electron density in the flatband region has been demonstrated through specific experiments of thermally stimulated capacitance and low‐temperature C‐V measurements performed on the sample cooled at different cooling rates.

Photoionization cross-section of the DX center in Te-doped AlxGa1−xSb

The photoionization of DX centers in Te-doped AlxGa1−xSb layers grown by molecular beam epitaxy is investigated by measuring the increase of the Hall free electron density after illumination by monochromatic light in the temperature range typical of the persistent photoconductivity (PPC) effect. The investigated samples have AlSb molar fractions in the 0.3⩽x⩽0.5 range and n-type doping in the 1017−1018 cm−3 range. An accurate investigation of the isothermal photoionization transients is performed to evidence features in the curve not directly related to the phenomenology of the DX center, the free electron density being influenced by the possible occupancy of other impurity levels. The transients show, in particular, an initial nonexponential behavior which is demonstrated as due to localization of a fraction of the photoexcited electrons into a nonmetastable impurity state which is responsible for the semiconductor-to-metal transition observed under the PPC regime. When this effect is accounted for, the ...

Concentration dependence of optical absorption in tellurium-doped GaSb

The optical absorption of molecular-beam-epitaxy-grown and Te-doped GaSb layers is measured in the spectral region of the fundamental absorption over a temperature range extending from 10 K to 300 K. In accordance with the Burstein-Moss description, a filling of the conduction-band states, resulting in a change of the shape and a shift of the absorption edge to higher energies, is observed in the absorption spectra of Te-doped n-type GaSb layers, with electron density ranging from 1.1 × 10 16 to 7.6 × 10 17 cm −3 at room temperature. A quantitative description of the Burstein-Moss effect is performed and the Fermi-level energy and the electron density in thevalley are obtained as a function of the temperature in two different ways: (i) by comparing absorption spectra of heavily doped and unintentionally or lightly doped GaSb samples; (ii) through a direct fit of absorption data performed in the framework of Kanes band model. The values of the Fermi level and of electron density in thevalley which have been optically obtained resulted in satisfactory agreement with those obtained from electrical measurements. The bandgap narrowing and the perturbation of the conduction-band density of states due to heavy doping in small-effective-mass semiconductors, such as GaSb, is considered in the framework of some current theoretical models.