Joseph W. Hemsky

Production and annealing of electron irradiation damage in ZnO

High-energy (>1.6 MeV) electrons create acceptors and donors in single-crystal ZnO. Greater damage is observed for irradiation in the [0001] direction (Zn face) than in the [0001] direction (O face). The major annealing stage occurs at about 300–325 °C, and is much sharper for defects produced by Zn-face irradiation, than for those resulting from O-face irradiation. The defects appear to have a chain character, rather than being simple, near-neighbor vacancy/interstitial Frenkel pairs. These experiments suggest that ZnO is significantly more “radiation hard” than Si, GaAs, or GaN, and should be useful for applications in high-irradiation environments, such as electronics in space satellites.

Electron-Irradiation-Induced Deep Level in n -Type GaN

Deep-level transient spectroscopy measurements of n-type GaN epitaxial layers irradiated with 1-MeV electrons reveal an irradiation-induced electron trap at EC−0.18 eV. The production rate is approximately 0.2 cm−1, lower than the rate of 1 cm−1 found for the N vacancy by Hall-effect studies. The defect trap cannot be firmly identified at this time.

Defect Models in Electron-Irradiated N-Type GaAs

1 MeV electron irradiation has been performed in degenerate, n‐type (n≂2×1017 cm−3), molecular beam epitaxial GaAs layers, and Hall effect measurements have been carried out during the irradiation in order to get accurate defect production data. The results have been fitted with statistical models, and are most consistent with the usual E1 (EC−0.045 eV) and E2 (EC−0.15 eV) levels being the (−/0) and (0/+) transitions of the As vacancy, respectively. Also, an acceptor well below EC−0.15 eV is produced at a much higher rate than that of E1 and E2.

Point Defect Characterization of GaN and ZnO

Abstract Point defects are created in bulk ZnO and epitaxial GaN by 1–2 MeV electron irradiation at 300 K, and are studied by temperature-dependent Hall effect, photoluminescence, and deep level transient spectroscopy measurements. The N vacancy is identified as a fairly shallow donor in GaN, whereas defect identifications in ZnO are uncertain at this time. Both materials, but especially ZnO, are quite resistant to displacement damage.

Incorporation of Silicon and Aluminum in Low-Temperature Molecular-Beam Epitaxial GaAs

The localized vibrational modes (LVMs) of silicon donor (SiGa) and aluminum isovalent (AlGa) impurities in molecular beam epitaxial GaAs layers grown at various temperatures are studied using the infrared absorption technique. It is found that the total integrated absorption of these impurities LVMs is decreased as the growth temperature decreases. This finding suggests a nonsubstitutional incorporation of Si and Al in GaAs layers grown at 200 °C. On the other hand, a subtitutional incorporation is obtained in GaAs layers grown at temperatures higher than 350 °C. A recovery of the SiGa LVMs in GaAs layers grown at 200 °C has not been achieved by thermal annealing.

In situ Hall-effect system for real-time electron-irradiation studies

A unique system capable of taking in situ Hall‐effect measurements during electron irradiation has been developed. The key element is a small, powerful rare‐earth magnet. Measurements can be taken while the electron beam is on, resulting in a considerable time savings and eliminating problems associated with mounting and demounting the sample. High resolution electron concentration and mobility versus fluence data are quickly and easily obtained, making possible detailed defect production rate studies as functions of energy and flux.

Identification of the 0.15 eV Donor Defect in Bulk GaAs

The well-known 0.15-eV Hall-effect center appearing in bulk, n-type GaAs quenches under IR illumination and recovers via an Auger-like process at a rate similar to the Auger rate of EL2. On the other hand, the 0.15-eV VA s -related center produced by 1-MeV electron irradiation does not quench at all. Based on these data and a detailed theoretical analysis by Baraff and Schluter, we argue that the bulk 0.15-eV center is related to the AsG a -VA s defect or a related complex.

Electron-Beam Modification of GaAs Surface-Potential - Measurement of Richardson Constant

The surface potential of GaAs is strongly modified in the presence of a high‐energy electron beam due to the creation of electron‐hole pairs in the depletion region and the subsequent drift of the holes to the surface where they neutralize surface states. This effect is modeled in terms of a parameter K=A*T2/Ib(dE/dz)η, where Ib is the beam current density, A* is the effective Richardson constant, dE/dz is the beam energy loss per unit length, and η−1 is the average energy required to create an electron‐hole pair. For the sample studied here, an 0.25‐μm layer with n≂3×1017 cm−3, we obtain a value K≂(7.5±0.8)×104 cm at T=296 K and Ib=0.33 μA/cm2, which gives A*≂0.44 A/cm2 K2. Although this value of A* is much lower than the theoretical estimate of 8 A/cm2 K2, it is in good agreement with other recent results.

Electrical and optical properties of as-grown and electron-irradiated ZnO

Large-diameter (up to 3-inch), n-type ZnO boules grown by a new vapor-phase transport method were studied by temperature-dependent Hall-effect (TDH) and photoluminescence (PL) measurements. From fits to the TDH data, the dominant donor has a concentration N/sub D/ of about 1/spl times/10/sup 17/ cm/sup -3/ and an energy of about 60 meV, close to the expected hydrogenic value, whereas the total acceptor concentration N/sub A/ is much lower, about 2/spl times/10/sup 15/ cm/sup -3/. The 2-K PL data include a series of at least seven sharp lines over the energy range 3.356-3.366 eV, and from polarization, magnetic-field, and annealing (up to 1000/spl deg/C) experiments, these lines are interpreted as donor-bound excitons associated with donor complexes having different spacings between the atomic species. Electron-irradiation experiments show an electrical and optical threshold, attributed to Zn displacements, at an electron bombardment energy of about 1.3 MeV. The irradiation creates acceptors and destroys shallow donors, but these effects are produced at much lower rates than those found in most other common semiconductor materials, such as Si, GaAs, and GaN.