M. Zazoui

Defects in electron irradiated GaInP

We present a characterization of the defects created by electron irradiation at room temperature in n‐type GaInP. Four electron traps, labeled E1–E4, and no hole traps have been detected using deep level transient spectroscopy in the temperature range 4–400 K. The corresponding energy levels and barriers associated with electron capture have been measured. The introduction rates, ranging from 4×10−3 to 0.4 cm−1, indicate that these defects are probably not primary defects but complexes resulting from the interaction of these primary defects between themselves or with impurities. This is not surprising, owing to the fact that defect annealing takes place below 300 K in InP.

Defects in epitaxial Si‐doped GaInP

We have characterized by capacitance‐voltage and deep level transient spectroscopy measurements the only defect detected in Si‐doped GaInP layers. This defect exhibits an ionization energy of 0.435 eV but is located only at ∼20 meV below the bottom of the conduction band. All its characteristics, i.e., energy level, apparent capture barrier, ionization energy, can be understood if the defect is a donor associated DX center. Its cross section for electron and hole capture have been measured. The effect of an electric field on the ionization energy confirms that the defect is indeed shallow and a donor.

Defects in electron‐irradiated GaAlAs alloys

Using deep‐level transient spectroscopy, we have characterized the energy levels, barriers for electron capture, and introduction rates of the defects introduced by electron irradiation in liquid‐phase epitaxy grown n‐type (Te)Ga1−xAlxAs layers of various alloy composition (x=0.25, 0.40, 0.60, and 0.80). We observed five defects which present various type behaviors: energy levels linked to the valence band or to the L conduction bands, constant barriers, or varying in a manner consistent with the band structure. These results are in agreement with the understanding obtained previously on electron‐induced defects in GaAs.

Recombination centers in electron‐irradiated Czochralski silicon solar cells

The defect responsible for the minority‐carrier lifetime in p‐type Czochralski silicon introduced by electron irradiation has been detected and characterized by deep‐level transient spectroscopy and spin‐dependent recombination. From the isotropic g value (2.0055), the defect is tentatively identified as a Si dangling bond originating from a vacancy cluster. Its energetic location in the gap is at 630 meV below the conduction band. The electron and hole cross sections and their variation with temperature have been determined, and found to account for the minority‐carrier lifetime of the material.

Defects in electron irradiated n‐type GaP

The characteristic ionization energies, barriers associated with capture, energy levels, and introduction rates of the various electron and hole traps introduced by electron irradiation in n‐type GaP are determined using deep level transient spectroscopy. The same traps are created after 4 or 300 K irradiation. Their introduction rates correspond to those expected for primary displacements. From the similarity to the case of GaAs, we conclude that the corresponding defects are intrinsic defects (isolated vacancies and vacancy interstitial pairs) associated with the P (electron traps) and Ga (hole traps) sublattices.

Recombination centers in Czochralski‐grown p‐Si

A sensitive deep‐level transient spectrometer operating in the range 300–600 K has been used to detect the defect responsible for the minority‐carrier lifetime in p‐type Czochralski‐grown Si. The characteristics of this defect (energy level, barrier for hole capture, hole and electron capture rates, and concentration) have been determined. This level is, as expected, located near the middle of the forbidden gap. We verified that it is indeed responsible for the lifetime by a comparison between the calculated value and the result of direct measurements. If this defect is an impurity, it could be manganese.

Electron transport through GaAlAs barriers in GaAs

We have analyzed electronic transport through a single, 200‐A‐thick, Ga0.74Al0.36As barrier embedded in GaAs. At low temperatures and high electric field, the Fowler–Nordheim regime is observed, indicating that the barrier acts as insulating layers. At higher temperatures the thermionic regime provides an apparent barrier height, decreasing with the field, which is equal to the expected band offset when extrapolated to zero field. However, for some samples, the current is dominated by the presence of electron traps located in the barrier. A careful analysis of the temperature and field behavior of this current allows to deduce that the mechanism involved is field‐enhanced emission from electron traps. The defects responsible are tentatively identified as DX centers, resulting from the contamination of the barrier by donor impurities.

Electrical characterization of AlAs layers and GaAs‐AlAs superlattices

We characterize by electrical techniques uniformly Si‐doped AlAs layers and short‐period GaAs‐AlAs superlattices grown in the same conditions by molecular‐beam epitaxy. Deep level transient spectroscopy shows that both the layers and the superlattices contain the DX center. The AlAs layers contain, in addition, a distribution of electron traps emitting in the temperature range 50–200 K. Using electron irradiation to introduce defects as probes we verified that the band structures of the superlattices we deduce are consistent with theoretical calculations using a widely accepted value of the band offset (67%). Finally, we observe that the DX center remains present, with the same ionization energy, when the energy position of the first Γ miniband varies while the first L miniband remains practically at the energetical position of the L band in the AlAs barriers. From this result we conclude that the DX center is linked to the L band and thus must be ascribed to an L effective‐mass state.

Defects in molecular beam epitaxy grown GaAlAs layers

Using deep level transient spectroscopy we characterized the shallow native traps in n‐type doped Ga1−xAlxAs layers (with x=0.30 and 0.36) grown by molecular beam epitaxy. A trap lying at 0.18 eV below the conduction band is detected which exists in large concentration within 0.2 μm from the surface and is responsible for the freeze out of free carriers at low temperatures.

The DX center in GaAsP alloys

Using Deep Level Transient Spectroscopy (DLTS), we have investigated the properties of the DX center in GaAsP for the whole range of alloy composition x. We have determined the variation of the defect characteristics (thermal ionization energy E i , barrier for electron capture B, and energy level location E DX ) versus x. From the relationship that exists between E i , B and E DX , and between B and Δ, the energy difference between the L band and the bottom of the conduction band, we deduce that electron emission and capture occur from and to the DX center via the L band in the same fashion as in GaAlAs alloys. A good fit of the variation of E i and B versus x is obtained in a model where the DX level is a donor state associated with the L band which is 200 meV deep, like in GaAlAs, as a result of intervalley mixing.