J. R. Sizelove

Electrical Properties of Bulk ZnO

Abstract Large-diameter (2-inch), n -type ZnO boules grown by a new vapor-phase transport method were investigated by the temperature-dependent Hall-effect technique. The 300-K Hall carrier concentration and mobility were about 6 × 10 16 cm −3 and 205 cm 2 V −1 s −1 , respectively, and the peak mobility (at 50 K) was about 2000 cm 2 V −1 s −1 , comparable to the highest values reported in the past for ZnO. The dominant donor had a concentration of about 1 × 10 17 cm −3 and an energy of about 60 meV, close to the expected hydrogenic value, whereas the total acceptor concentration was much lower, about 2 × 10 15 cm −3 . Photoluminescence measurements confirm the high quality of the material.

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.

As-doped p-type ZnO produced by an evaporation∕sputtering process

Strongly p-type ZnO is produced by the following sequence of steps: (1) evaporation of Zn3As2 on a fused-quartz substrate at 350°C; and (2) sputtering of ZnO with substrate held at 450°C. The electrical characteristics include: resistivity of 0.4Ωcm, a mobility of 4cm2∕Vs, and a hole concentration of about 4×1018cm−3. This resistivity is among the best (lowest) ever reported for p-type ZnO. Secondary-ion mass spectroscopic analysis gives an average As concentration about 5×1019cm−3, and a simple one-band fit of the temperature-dependent mobility curve yields an acceptor concentration of about 9×1019cm−3. This is strong evidence that the p-type dopant involves As, although it is not clear whether the acceptor is simply AsO or the recently suggested AsZn−2VZn.

On the nitrogen vacancy in GaN

The dominant electrically active defect produced by 0.42 MeV electron irradiation in GaN is a 70 meV donor. Since only N-sublattice displacements can be produced at this energy, and since theory predicts that the N interstitial is a deep acceptor in n-type GaN, we argue that the 70 meV donor is most likely the isolated N vacancy. The background shallow donors, in the 24–26 meV range, actually decrease in concentration, probably due to interactions with mobile N interstitials that are produced by the irradiation. Thus, the recent assignment of a photoluminescence (PL) line as an exciton bound to a 25 meV N-vacancy donor is incompatible with our results. Moreover, we do not observe that PL line in our sample.

Predicted maximum mobility in bulk GaN

A 300 K bulk (three-dimensional) mobility of 1245 cm2/V s has been measured in free-standing GaN. Temperature-dependent Hall-effect data on this particular sample are fitted to obtain unknown lattice-scattering parameters, as well as shallow donor (ND) and acceptor (NA) concentrations, which are ND=6.7×1015 and NA=1.7×1015 cm−3. Realistic values of the maximum mobility attainable in bulk GaN are then obtained by assuming two-orders-of-magnitude lower values of ND and NA, leading to a maximum 300 K mobility of 1350 cm2/V s, and a maximum 77 K mobility of 19 200 cm2/V s.

Accurate mobility and carrier concentration analysis for GaN

Abstract Temperature-dependent mobility μ and carrier concentration n data are simultaneously fitted in a high-quality, n-type GaN layer with wurtzite structure grown by metalorganic chemical vapor deposition. The mobility fit is found to be very important for obtaining the correct acceptor concentration, N A ⋍ 1.1 ± 0.2 × 10 17 cm −3 . Using this range of NA, a fit to the n vs T curve, corrected for Hall r-factor, gives two donor levels of concentration (energy): 2.7 ± 0.2 × 1017 cm−3 (7.5 ± 1.5 meV) and 1.5 ± 0.2 × 1017 cm−3 (58 ± 2 meV), respectively. However, the n vs T curve by itself is inadequate for an accurate determination of NA; in this case, acceptable fits can be obtained for NA ranging from 0 to 1.2 × 1017 cm−3.

Annealing dynamics of molecular‐beam epitaxial GaAs grown at 200 °C

By separating a 2‐μm‐thick molecular‐beam‐epitaxial GaAs layer grown at 200 °C from its 650‐μm‐thick substrate, we have been able to obtain accurate Hall‐effect and conductivity data as functions of annealing temperature from 300 to 600 °C. At a measurement temperature of 300 K, analysis confirms that hopping conduction is much stronger than band conduction for all annealing temperatures. However, at higher measurement temperatures (up to 500 K), the band conduction becomes comparable, and a detailed analysis yields the donor and acceptor concentrations and the donor activation energy. Also, an independent absorption study yields the total and charged AsGa concentrations. Comparisons of all of these quantities as a function of annealing temperature TA show a new feature of the annealing dynamics, namely, that the dominant acceptor (probably VGa related) strongly decreases and then increases as TA is increased from 350 to 450 °C. Above 450 °C, ND, NA, and [AsGa] all decrease, as is known from previous studies.

Semiconducting/Semi-Insulating Reversibility in Bulk GaAs

Bulk, liquid‐encapsulated Czochralski GaAs may be reversibly changed from semiconducting (ρ∼1 Ω cm) to semi‐insulating (ρ∼107 Ω cm) by slow or fast cooling, respectively, following a 5 h, 950 °C soak in an evacuated quartz ampoule. This effect has been studied by temperature‐dependent Hall‐effect, photoluminescence, infrared absorption, mass spectroscopy, and deep level transient spectroscopy measurements. Except for boron, the samples are very pure, with carbon and silicon concentrations less than 3×1014 cm−3. Donor and acceptor concentrations, on the other hand, are in the mid 1015 cm−3 range, which means that the compensation is primarily determined by native defects, not impurities. A tentative model includes a donor at EC−0.13 eV, attributed to VAs−AsGa, and an acceptor at EV+0.07 eV, attributed to VGa−GaAs.

Donor and acceptor concentrations in molecular beam epitaxial GaAs grown at 300 and 400 °C

The first Hall‐effect measurements on molecular beam epitaxial GaAs layers grown at the low temperatures of 300 and 400 °C are reported. Two independent methods were used to determine donor ND and acceptor NA concentrations and activation energy ED0, with the following combined results: ND≂3±1×1018, NA≂1.5±1×1017 cm−3, and ED0=0.645±0.009 eV for the 300 °C layer; ND≂2±1×1017, NA≂7±3×1016 cm−3, and ED0=0.648±0.003 eV for the 400 °C layer. Thus, the deep donor is not the expected EL2, which has ED0=0.75±0.01 eV.

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.