Chun-Yuan Huang

The effect of thermal annealing on the Ni/Au contact of p-type GaN

In this study, the Ni/Au layers prepared by electron beam evaporation and thermal alloying were used to form Ohmic contacts on p-type GaN films. Before thermal alloying, the current–voltage (I–V) characteristic of Ni/Au contact on p-type GaN film shows non-Ohmic behavior. As the alloying temperature increases to 700u2009°C, the I–V curve shows a characteristic of Ohmic contact. The Schottky barrier height reduction may be attributed to the presence of Ga–Ni and Ga–Au compounds, such as Ga4Ni3,u2002Ga3Ni2, GaAu, and GaAu2, at the metal-semiconductor interface. The diffusing behavior of both Ni and Au have been studied by using Auger electron spectroscopy and Rutherford backscattering spectrometry. In addition, x-ray diffraction measurements indicate that the Ni3N and Ga4Ni3 compounds were formed at the metal-semiconductor interface.

DEEP ULTRAVIOLET ENHANCED WET CHEMICAL ETCHING OF GALLIUM NITRIDE

We report a study of the ultraviolet (UV) irradiation effects on the wet chemical etching of unintentionally doped n-type gallium nitride (GaN) layers grown on sapphire substrates. When illuminated with a 253.7 nm mercury line source, etching of GaN is found to take place in aqueous phosphorus acid (H3PO4) and potassium hydroxide (KOH) solutions of pH values ranging from −1 to 2 and 11 to 15, respectively. Formation of gallium oxide is observed on GaN when illuminated in dilute H3PO4 and KOH solutions. These results are attributed to a two-step reaction process upon which the UV irradiation is shown to enhance the oxidative dissolution of GaN.

Photoenhanced wet oxidation of gallium nitride

We investigate the photo-oxidation process and the corresponding passivation effects on the optical properties of unintentionally doped n-type gallium nitride (GaN). When illuminated with a 253.7 nm mercury line source, oxidation of GaN is found to take place in aqueous phosphorus acid solutions with pH values ranging from 3 to 4. At room temperature, the photo-oxidation process is found reaction-rate limited and has a peak value of 224 nm/h at pH=3.5. Compared with the as-grown GaN layers, threefold enhancement in the photocurrent and photoluminescence response are observed on the oxidized GaN surfaces. These results are attributed to the surface passivation effects due to the deep ultraviolet-enhanced wet oxidation on GaN.

PHOTOLUMINESCENCE SPECTROSCOPY OF MG-DOPED GAN

We have grown Mg-doped GaN films by metalorganic chemical vapor deposition with various CP2Mg flow rates. After 750u2009°C postgrowth annealing, p-type GaN films with carrier concentrations and mobilities about 2×1017/cm3 and 10 cm2/Vu2009s, respectively, have been achieved. A dominant photoluminescence (PL) line around 2.9 eV was observed at room temperature. By studying the dependence of PL on excitation density at 20 K, the emission line around 2.95 eV can be attributed to a donor-to-acceptor pair transition rather than to a conduction band-to-impurity transition involving the Mg-related deep level. We suggest that the DAP transition line is caused by a Mg related deep level at about 510 meV above the valence band. It is much deeper than the acceptor level at 250 meV commonly produced by the Mg dopants.

All-organic hot-carrier triodes with thin-film metal base

In this letter, the authors investigate the promising vertical-type triodes based on small organic molecules and the related hot-carrier transport. The devices show transistorlike characteristics, in which output current can be modulated by demanding different input currents on their thin metal base electrodes. By using pentacene for the channel layer material and N,N′-di(naphthalen-l-yl)-N,N′-diphenyl-benzidine for the carrier energy-enhancing layer, the vertical-type hot carrier triodes exhibit a good current saturation with current gain of 2.38 for both the common-base and common-emitter configurations. The mechanism of operation is proposed and examined by the basic electrical measurements.

Temperature dependence of carrier dynamics for InAs∕GaAs quantum dot infrared photodetectors

Temperature-dependent micro-photoluminescence (μ-PL) spectra and the spectral response for the 30-period undoped InAs∕GaAs quantum-dot infrared photodetectors (QDIPs) are investigated. The competition of different transition levels within the spectra is observed. Also observed is the influence of dot size and density on the μ-PL characteristics. Consistently, the mid-infrared spectral response for the fabricated QDIPs exhibit the same energy position as the shifted PL spectra relative to the energy of wetting layer, which indicates the multi-transition mechanisms responsible for the QDIP spectral response.

Formation and Characterization of 1.5-Monolayer Self-Assembled InAs/GaAs Quantum Dots Using Postgrowth Annealing

In this paper, we report that low-density InAs/GaAs quantum dots (QDs) can be formed by postgrowth annealing the samples with 1.5-monolayer (ML) InAs coverage, which is thinner than the critical layer thickness for the Stranski-Krastanov growth. The annealing procedure was performed immediately after the deposition of the InAs layer. The effects of annealing time and annealing temperature on the dot density, dot size, and optical characteristics of the QDs were investigated. The optimum annealing conditions to obtain low-density QDs are longer than 60 s and higher than 500degC . Meanwhile, no luminescence can be observed for the wetting-layer, which may suggest that the postgrowth annealing will make the wetting layer thinner and thus reduce the effects of wetting layer on carrier relaxation and recombination. On the other hand, we observe that a decrease of the PL intensity at the annealing conditions of 60 s and 515degC , which is possibly due to the increasing surface dislocations resulted from the In adatom desorption at higher annealing temperature.

Transport mechanisms and the effects of organic layer thickness on the performance of organic Schottky diodes

Experimental results of static and dynamic characteristics for single-layer hole-only devices based on copper phthalcyanine (CuPc) and pentacene are observed in this article. The contribution to injection currents from electrode has been investigated by varying the thickness of the organic film. From the observation of current density versus bias voltage (J-V) characteristics, it is concluded that the space-charge-limited conductivity is the dominant transport mechanism for the organic Schottky diodes. Accordingly, an increase of the organic layer thickness will increase the trapping energy level. However, even with the thin CuPc film down to 50nm, the dynamic cut-off frequency of the device is still limited to 150Hz. Low hole mobility and large active area of the device are responsible for the phenomenon. Dramatic enhancement of cut-off frequency up to 11kHz can be obtained for the pentacene-based Schottky diodes.

Self-ordered InGaAs quantum dots grown at low growth rates

This study explores the effects of the growth rate on InGaAs∕GaAs quantum dots (QDs) in producing ordered QD arrays. Surface morphological observations reveal that the dot density decreases as the growth rate increases and the QDs can be gradually self-ordered in the [11¯0] direction. The threshold growth rate for one-dimensional self-ordered QD arrays is 0.054μm∕h. This phenomenon is attributed to the preferential QD nucleation at the local strain maximum which is at the edge of the elongated step bunch. After the step bunches that elongated along [11¯0] become the main feature on the wetting layer surface, the accumulated strain field on both step edges favors the nucleation of QDs. However, the concurrent decrease in QD size and the thinning of the wetting layer may be associated with the desorption of In and Ga adatoms and the lateral mass transport, respectively.