K. S. Stevens

Photoconductive ultraviolet sensor using Mg‐doped GaN on Si(111)

This work characterizes a GaN:Mg on silicon ultraviolet photodetector with a cutoff at 3.3 eV and a responsivity of 12 A/W at 4 V bias for optical intensities on the order of 1 W/m2 and below. A weak photovoltaic response is also reported. The photocurrent is nearly linear versus optical intensity for up to 10 W/m2. The responsivity increases nearly linearly with applied voltage up to 8 V, then the increase slows toward saturation. To explain this high responsivity in a direct gap semiconductor, it is hypothesized that holes are captured at either compensated Mg deep acceptor sites or Mg‐related trap/recombination centers, resulting in a greatly prolonged electron free‐carrier lifetime.

Microstructure and photoluminescence of GaN grown on Si(111) by plasma‐assisted molecular beam epitaxy

Wurtzite GaN films on AlN buffer layers were grown on Si(111) by plasma‐assisted molecular beam epitaxy. High resolution x‐ray diffraction and scanning electron microscope studies indicate that the mosaic disorder decreases with increasing film thickness and increasing growth temperature. The grain size increases with the growth temperature. The best (0002) diffraction peak full width at half‐maximum is 22 arcmin for a film 1.7 μm thick. Prominent low‐temperature exciton luminescence is observed at 3.46 eV. The plasma I‐V characteristics are measured with a Langmuir probe near the growth position and analyzed to extract the nitrogen ion density and energy for the growth conditions used.

Microstructure of AlN on Si (111) grown by plasma‐assisted molecular beam epitaxy

We have grown monocrystalline AlN on Si (111) substrates over the temperature range 550–900 °C using electron cyclotron resonance plasma assisted molecular beam epitaxy. The best (0002) peak omega rocking curve full width at half‐maximum value obtained was 26 min for a film deposited at 900 °C. All films nominally displayed the AlN[0001]∥Si[111] orientation. The exact angle between AlN[0001] and Si[111] decreased from 2.1° to 1.1° and the (0002) peak widths improved with increasing substrate temperature. Mosaic‐type disorder was shown by high resolution x‐ray diffraction to be the dominant cause of the ω rocking curve peak full widths.

DEMONSTRATION OF A SILICON FIELD-EFFECT TRANSISTOR USING ALN AS THE GATE DIELECTRIC

As a precursor to cointegration of blue and ultraviolet light emitting nitrides with silicon microelectronics, an AlN on Si(111) metal‐insulator‐semiconductor field effect transistor has been fabricated and characterized. Current–voltage data show the threshold voltage and transconductance to be −0.8 V and 6 ms/mm, respectively, for a 50 nm insulator thickness. The ideal threshold voltage is 0.09 V. From the transconductance, a channel mobility of 45 cm2/V s is inferred. Capacitance versus voltage (C–V) measurements on a 210 nm film showed a flatband voltage of −4.2 V, corresponding to 8×1011 cm−2 trapped charges at the interface. Comparison of the experimental high‐frequency (1 MHz) C–V plot to the theoretical plot indicates the presence of fast interface states. High‐resolution transmission electron microscopy studies showed the AlN/Si(111) interface to be surprisingly well ordered in places, with the large lattice mismatch accommodated via a quasiperiodic array of misfit dislocations.

Growth of group III nitrides on Si(111) by plasma‐assisted molecular beam epitaxy

Wurtzitic single‐crystal GaN and polycrystalline (columnar) InGaN have been grown on the Si(111) face in an electron cyclotron resonance plasma‐assisted molecular beam epitaxy process. Reflection high‐energy electron diffraction shows registry of the nitride basal‐plane triangular lattice with respect to the Si(111) substrate triangular network. Plan‐view transmission electron microscopy images reveal crystalline or polycrystalline GaN structure depending on growth temperature. High‐resolution x‐ray rocking curves of the (0002) peak were as narrow as 30 min for a 0.6‐μm GaN film grown on a thin AlN buffer layer at 750 °C.

Microstructure and composition of InAsN alloys grown by plasma-source molecular beam epitaxy

A series of growths conducted at different substrate temperatures and V:III ratios is analyzed by x-ray diffraction to determine composition. A metastable InAsN alloy plus pure InAs are obtained for temperatures in the range 450–500 °C and total V:III ratio of approximately unity. The InAs fraction in the alloy phase increases at lower temperatures, the maximum observed is 13%. For higher temperatures or higher V:III ratio only separated phases of InAs and InN are found.

Feasibility of the synthesis of AlAsN and GaAsN films by plasma-source molecular-beam epitaxy

Possible experimental realizations of AlAsN and GaAsN alloys are explored. Growths were conducted to test whether the pseudobinary alloys III-AsxN1 − x could be formed by providing As2 and excess Group III fluxes in a plasma-source nitride epitaxial growth process based on Si(1 1 1) substrates. Incorporation of several atomic percent of As occurs at typical III-N growth temperatures. However, X-ray and transmission electron microscopy analysis show that disordered phases are present and that the incorporated As is not purely substitutional on N sublattice sites. A pronounced surfactant effect of As in the growth of GaN is demonstrated, with the rms surface roughness decreasing one order of magnitude. Another approach, using “digital” alloys of GaAsN formed by alternating beam epitaxy on GaAs(1 0 0) has demonstrated incorporation of several atomic percent of N in GaAs. High-resolution X-ray rocking curves and TEM images of the digital alloy are provided.

Influence of substrate electrical bias on the growth of GaN in plasma‐assisted epitaxy

A sample bias on the order of −15 V arises during electron cyclotron resonance plasma‐assisted epitaxy of GaN on insulating substrates. In contrast, conducting substrates enable growth under zero or positive bias conditions. Surface morphology of GaN/Si(111) is progressively smoother with increasing bias (−10 to +10 V) for a microwave power of 30 W. Emission spectroscopy of the plasma suggests that both N and N+2 are produced from collisions involving N*2 and that for low power (10 W), the N and N+2 production is limited by the N*2 abundance. In contrast, at higher power (50 W), ion and atom production appears limited by the electron collision frequency. Electrostatic probe measurements near the growth stage indicate that the ion energy has a minimum of about 6 eV, attained at low plasma power or at medium power (30 W) with a positive substrate bias. Mg‐doped films produced at 10 W and zero bias are highly resistive. p‐n junction diodes formed by Si‐doped GaN grown on Mg‐doped GaN on p‐type Si are operate...

Analysis and optimization of the electron cyclotron resonance plasma for nitride epitaxy

Abstract Mg-doped GaN films are grown on Si(111) substrates using plasma-assisted molecular beam epitaxy. Plasma diagnosis and analysis predict operating conditions that minimize the ion bombardment during the growth. The ion exposure strongly affects the photoluminescence intensity associated with the Mg acceptor level in GaN. The drift mechanism of the nitrogen ions from the electron cyclotron resonance region is discussed. By minimizing the ion drift during growth, intense photoluminescence is obtained from the films.

Material and Device Characteristics of MBE-Grown GaN Using a New rf Plasma Source

A new rf plasma nitrogen source has been characterized for growth of GaN on basal-plane sapphire by molecular beam epitaxy. For rf power of 500 W and N{sub 2} flow rate of 2 sccm, a maximum GaN growth rate of 0.80 {micro}m/hr is obtained,m implying a source efficiency greater than 5%. It is found that the GaN surface roughness is extremely sensitive to V:III ratio near unity and independent of growth rate in the range 0.3--0.8 {micro}m/hr. Roughness as small as 1.0 nm (rms) is measured by atomic-force microscopy. Microstructure of the high-growth-rate films is similar to other GaN films, as observed in cross-section transmission electron microscope images. The electroluminescence spectra from homojunction light-emitting diodes exhibit a band of near-ultraviolet emissions corresponding to the energy separation of the intentional donor and acceptor levels on the two sides of the junction. The intensity of these emissions relative to the visible spectrum increases with drive current density, implying saturation of deep trap levels responsible for the visible light output.