R. Dahal

InGaN/GaN multiple quantum well solar cells with long operating wavelengths

We report on the fabrication and photovoltaic characteristics of InGaN solar cells by exploiting InGaN/GaN multiple quantum wells (MQWs) with In contents exceeding 0.3, attempting to alleviate to a certain degree the phase separation issue and demonstrate solar cell operation at wavelengths longer than previous attainments (>420 nm). The fabricated solar cells based on In0.3Ga0.7N/GaN MQWs exhibit an open circuit voltage of about 2 V, fill factor of about 60%, and an external efficiency of 40% (10%) at 420 nm (450 nm).

InGaN/GaN multiple quantum well concentrator solar cells

We present the growth, fabrication, and photovoltaic characteristics of Inx Ga1−xN/GaN(x∼0.35) multiple quantum well solar cells for concentrator applications. The open circuit voltage, short circuit current density, and solar-energy-to-electricity conversion efficiency were found to increase under concentrated sunlight. The overall efficiency increases from 2.95% to 3.03% when solar concentration increases from 1 to 30 suns and could be enhanced by further improving the material quality.

Epitaxially grown semiconducting hexagonal boron nitride as a deep ultraviolet photonic material

Hexagonal boron nitride (hBN) has emerged as an important material for various device applications and as a template for graphene electronics. Low-dimensional hBN is expected to possess rich physical properties, similar to graphene. The synthesis of wafer-scale semiconducting hBN epitaxial layers with high crystalline quality and electrical conductivity control has not been achieved but is highly desirable. Large area hBN epitaxial layers (up to 2 in. in diameter) were synthesized by metal organic chemical vapor deposition. P-type conductivity control was attained by in situ Mg doping. Compared to Mg-doped wurtzite AlN, which possesses a comparable energy band gap (∼6 eV), dramatic reductions in Mg acceptor energy level and P-type resistivity (by about six to seven orders of magnitude) have been realized in hBN epilayers. The ability of conductivity control and wafer-scale production of hBN opens up tremendous opportunities for emerging applications, ranging from revolutionizing p-layer approach in III-ni...

Dielectric strength, optical absorption, and deep ultraviolet detectors of hexagonal boron nitride epilayers

Hexagonal boron nitride (hBN) epilayers have been synthesized by metal organic chemical vapor deposition and their dielectric strength, optical absorption, and potential as a deep ultraviolet (DUV) detector material have been studied. Based on the graphene optical absorption concept, the estimated band-edge absorption coefficient of hBN is about 7 × 105/cm, which is more than 3 times higher than the value for wurtzite AlN (∼2 × 105 /cm). The dielectric strength of hBN epilayers exceeds that of AlN and is greater than 4.4 MV/cm based on the measured result for an hBN epilayer released from the host sapphire substrate. The hBN epilayer based DUV detectors exhibit a sharp cut-off wavelength around 230 nm, which coincides with the band-edge photoluminescence emission peak and virtually no responses in the long wavelengths. Based on the present study, we have identified several advantageous features of hBN DUV photodetectors: (1) low long wavelength response or high DUV to visible rejection ratio; (2) requirin...

Epitaxial growth and demonstration of hexagonal BN/AlGaN p-n junctions for deep ultraviolet photonics

Recent advances in epitaxial growth and demonstration of p-type conductivity in hexagonal boron nitride (hBN) epilayers represent an exceptional opportunity to revolutionize p-layer approach and overcome the intrinsic problem of low p-type conductivity in Al-rich AlGaN for deep ultraviolet (DUV) device applications. Nevertheless, the ability of epitaxial growth of hBN on AlGaN is a prerequisite for the incorporation of p-type hBN in AlGaN DUV device structures. We report on the epi-growth of hBN on Al-rich AlGaN/AlN/Al2O3 templates using metal organic chemical vapor deposition. X-ray diffraction measurement revealed a 2θ peak at 26.5° which indicates that the BN epilayers are hexagonal and consist of a single phase. Mg doped hBN epilayers were also grown on highly insulating AlN and n-type AlGaN templates with an attempt to demonstrate hBN/AlGaN p-n junctions. Mg doped hBN epilayers grown on insulating templates were p-type with an in-plane resistivity of ∼2.3 Ω cm. Diode behavior in the p-n structures of...

1.54 μm emitters based on erbium doped InGaN p-i-n junctions

We present here on the growth, fabrication and electroluminescence (EL) characteristics of light emitting diodes (LEDs) based on Er-doped InGaN active layers. The p-i-n structures were grown using metal organic chemical vapor deposition and processed into 300×300 μm2 mesa devices. The LEDs exhibit strong emissions at 1.0 and 1.54 μm, due to Er intra-4f transitions, under forward bias conditions. The emitted EL intensity increases with applied input current without exhibiting saturation up to 70 mA. The integrated power over the near infrared emission, measured at room temperature from the top of a bare chip, is about 2 μW. The results represent a significant advance in the development of current injected, chip-scale emitters and waveguide amplifiers based on Er doped semiconductors.

Erbium-doped GaN optical amplifiers operating at 1.54 μm

Strip optical waveguides based on erbium (Er)-doped AlGaN/GaN:Er/AlGaN heterostructures have been fabricated and characterized in the optical communication wavelength window near 1.54 μm. The propagation loss of these waveguide amplifiers have been measured at 1.54 μm and found to be 3.5 cm−1. Moreover, the optical amplification properties of the waveguides were measured using a signal input at 1.54 μm and a broadband GaN light-emitting diode at 365 nm as pump source. A relative signal enhancement of ∼8 cm−1 was observed. The implications of such devices in photonic integrated circuits for optical communications are discussed.

Band-edge transitions in hexagonal boron nitride epilayers

Hexagonal boron nitride (hBN) epilayers have been synthesized on sapphire substrates by metal-organic chemical vapor deposition (MOCVD). These MOCVD grown epilayers exhibit highly efficient band-edge photoluminescence (PL) emission lines centered at around 5.5 eV. The results represent a remarkable improvement over the optical qualities of hBN films synthesized by different methods in the past. It was observed that the emission of hBN at 10 K is about 500 times stronger than that of high quality AlN epilayers. Polarization-resolved PL spectroscopy revealed that hBN epilayers are predominantly a surface emission material, in which the band-edge emission with electric field perpendicular to the c-axis (E⇀emi⊥c⇀) is about 1.7 times stronger than the component along the c-axis (E⇀emi‖c⇀). This is in contrast to AlN, in which the band-edge emission is known to be polarized along the c-axis, (E⇀emi‖c⇀). Time-resolved PL measurements revealed a decay lifetime of around 4.3 ns at 10 K for the dominant band-edge t...

Realizing InGaN monolithic solar-photoelectrochemical cells for artificial photosynthesis

InGaN alloys are very promising for solar water splitting because they have direct bandgaps that cover almost the whole solar spectrum. The demonstration of direct solar-to-fuel conversion without external bias with the sunlight being the only energy input would pave the way for realizing photoelectrochemical (PEC) production of hydrogen by using InGaN. A monolithic solar-PEC cell based on InGaN/GaN multiple quantum wells capable to directly generate hydrogen gas under zero bias via solar water splitting is reported. Under the irradiation by a simulated sunlight (1-sun with 100 mW/cm2), a 1.5% solar-to-fuel conversion efficiency has been achieved under zero bias, setting a fresh benchmark of employing III-nitrides for artificial photosynthesis. Time dependent hydrogen gas production photocurrent measured over a prolonged period (measured for 7 days) revealed an excellent chemical stability of InGaN in aqueous solution of hydrobromic acid. The results provide insights into the architecture design of using ...

High quality AlN for deep UV photodetectors

We have prepared large-area, 0.50×0.55mm2, metal-semiconductor-metal photodetectors based on AlN layers with different density of inversion domains (IDs). AlN layers were grown on (0001) sapphire substrates using gas source molecular beam epitaxy. The introduction of AlN∕GaN short period superlattices after growth of AlN nucleation layer yields significant reduction in the ID density. Photodetectors with ID density of 106cm−2 exhibit a very low dark current of 0.5fA at zero bias, which remains below 50fA up to a bias of ±30V. The peak responsivity of 0.08A∕W was obtained at a wavelength of ∼202nm.