Jossue Montes

Nonpolar and semipolar InGaN/GaN multiple-quantum-well solar cells with improved carrier collection efficiency

We demonstrate the nonpolar and semipolar InGaN/GaN multiple-quantum-well (MQW) solar cells grown on the nonpolar m-plane and semipolar ( 20 2 ¯ 1 ) plane bulk GaN substrates. The optical properties and photovoltaic performance of the nonpolar and semipolar InGaN solar cells were systematically studied, and the results were compared to the conventional polar c-plane devices. The absorption spectra, current density–voltage (J–V) characteristics, external quantum efficiency (EQE), and internal quantum efficiency (IQE) were measured for nonpolar m-plane, semipolar ( 20 2 ¯ 1 ) plane, and polar c-plane InGaN/GaN MQW solar cells. Nonpolar m-plane InGaN/GaN MQW solar cells showed the best performance across all devices, with a high open-circuit voltage of 2.32 V, a low bandgap-voltage offset of 0.59 V, and the highest EQE and IQE. In contrast, the polar c-plane device showed the lowest EQE despite the highest absorption spectra. This huge difference is attributed to the better carrier transport and collection o...

Active tracking system for visible light communication using a GaN-based micro-LED and NRZ-OOK

Visible light communication (VLC) holds the promise of a high-speed wireless network for indoor applications and competes with 5G radio frequency (RF) system. Although the breakthrough of gallium nitride (GaN) based micro-light-emitting-diodes (micro-LEDs) increases the -3dB modulation bandwidth exceptionally from tens of MHz to hundreds of MHz, the light collected onto a fast photo receiver drops dramatically, which determines the signal to noise ratio (SNR) of VLC. To fully implement the practical high data-rate VLC link enabled by a GaN-based micro-LED, it requires focusing optics and a tracking system. In this paper, we demonstrate an active on-chip tracking system for VLC using a GaN-based micro-LED and none-return-to-zero on-off keying (NRZ-OOK). Using this novel technique, the field of view (FOV) was enlarged to 120° and data rates up to 600 Mbps at a bit error rate (BER) of 2.1×10-4 were achieved without manual focusing. This paper demonstrates the establishment of a VLC physical link that shows enhanced communication quality by orders of magnitude, making it optimized for practical communication applications.

Characterizations of nonlinear optical properties on GaN crystals in polar, nonpolar, and semipolar orientations

We report the basic nonlinear optical properties, namely, two-photon absorption coefficient ( β), three-photon absorption coefficient ( γ), and Kerr nonlinear refractive index ( n kerr), of GaN crystals in polar c-plane, nonpolar m-plane, and semipolar ( 20 21 ¯) plane orientations. A typical Z-scan technique was used for the measurement with a femtosecond Ti:S laser from wavelengths of 724 nm to 840 nm. For the two-photon absorption coefficient ( β), similar values were obtained for polar, nonpolar, and semipolar samples, which are characterized to be ∼0.90 cm/GW at 724 nm and ∼0.65 cm/GW at 730 nm for all the three samples. For the Kerr nonlinear refractive index ( n kerr), self-focusing features were observed in this work, which is different from previous reports where self-defocusing features were observed on GaN in the visible and near-UV spectral regions. At 724 nm, n kerr was measured to be ∼2.5 0 × 10 − 14 cm 2 / W for all three samples. Three-photon absorption coefficients ( γ) were also determ...

Reliability analysis of InGaN/GaN multi-quantum-well solar cells under thermal stress

We investigate the thermal stability of InGaN solar cells under thermal stress at elevated temperatures from 400 °C to 500 °C. High Resolution X-Ray Diffraction analysis reveals that material quality of InGaN/GaN did not degrade after thermal stress. The external quantum efficiency characteristics of solar cells were well-maintained at all temperatures, which demonstrates the thermal robustness of InGaN materials. Analysis of current density–voltage (J–V) curves shows that the degradation of conversion efficiency of solar cells is mainly caused by the decrease in open-circuit voltage (Voc), while short-circuit current (Jsc) and fill factor remain almost constant. The decrease in Voc after thermal stress is attributed to the compromised metal contacts. Transmission line method results further confirmed that p-type contacts became Schottky-like after thermal stress. The Arrhenius model was employed to estimate the failure lifetime of InGaN solar cells at different temperatures. These results suggest that wh...

Low loss GaN waveguides at the visible spectral wavelengths for integrated photonics applications

We perform comprehensive studies on the fundamental loss mechanisms in III-nitride waveguides in the visible spectral region. Theoretical analysis shows that free carrier loss dominates for GaN under low photon power injection. When optical power increases, the two photon absorption loss becomes important and eventually dominates when photon energy above half-bandgap of GaN. When the dimensions of the waveguides reduce, the sidewall scattering loss will start to dominate. To verify the theoretical results, a high performance GaN-on-sapphire waveguide was fabricated and characterized. Experimental results are consistent with the theoretical findings, showing that under high power injection the optical loss changed significantly for GaN waveguides. A low optical loss ~2 dB/cm was achieved on the GaN waveguide, which is the lowest value ever reported for the visible spectral range. The results and fabrication processes developed in this work pave the way for the development of III-nitride integrated photonics in the visible and potentially ultraviolet spectral range for nonlinear optics and quantum photonics applications.

Characterizations of the nonlinear optical properties for (010) and (2- 01) beta-phase gallium oxide

We report, for the first time, the characterizations on optical nonlinearities of beta-phase gallium oxide (β-Ga2O3), where both (010) β-Ga2O3 and (2¯01) β-Ga2O3 were examined for two-photon absorption coefficient, Kerr nonlinear refractive index, and their polarization dependence. The wavelength dependence of two-photo absorption coefficient and Kerr nonlinear refractive index were also estimated by a widely used analytical model. β-Ga2O3 exhibits a two photon absorption (TPA) coefficient of 1.2 cm/GW for (010) β-Ga2O3 and 0.6 cm/GW for (2¯01) β-Ga2O3. The Kerr nonlinear refractive index is -2.1 × 10-15 cm2/W for (010) β-Ga2O3 and -2.9 × 10-15 cm2/W for (2¯01) β-Ga2O3. In addition, β-Ga2O3 shows stronger in-plane nonlinear optical anisotropy on (2¯01) plane than on (010) plane. Compared with GaN, TPA coefficient of β-Ga2O3 is 20 times smaller, and the Kerr nonlinear refractive index of β-Ga2O3 is also found to be 4-5 times smaller. These results indicate that β-Ga2O3 have the potential for ultra-low loss waveguides and ultra-stable resonators and integrated photonics, especially in UV and visible wavelength spectral range.

Energy band engineering of InGaN/GaN multi-quantum-well solar cells via AlGaN electron- and hole-blocking layers

In this paper, we perform a comprehensive study on energy band engineering of InGaN multi-quantum-well (MQW) solar cells using AlGaN electron- and hole-blocking layers. InGaN MQW solar cells with AlGaN layers were grown by metalorganic chemical vapor deposition, and high crystal quality was confirmed by high resolution X-ray diffraction measurements. Time-resolved photoluminescence results showed that the carrier lifetime on the solar cells with AlGaN layers increased by more than 40% compared to that on the reference samples, indicating greatly improved carrier collections. The illuminated current-density (J–V) measurements further confirmed that the short-circuit current density (Jsc) of the solar cells also benefited from the AlGaN layer design and increased 46%. At room temperature, the InGaN solar cells with AlGaN layers showed much higher power conversion efficiency (PCE), by up to two-fold, compared to reference devices. At high temperatures, these solar cells with AlGaN layers also delivered superior photovoltaic (PV) performance such as PCE, Jsc, and fill factor than the reference devices. These results indicate that band engineering with AlGaN layers in the InGaN MQW solar cell structures can effectively enhance the carrier collection process and is a promising design for high efficiency InGaN solar cells for both room temperature and high temperature PV applications.In this paper, we perform a comprehensive study on energy band engineering of InGaN multi-quantum-well (MQW) solar cells using AlGaN electron- and hole-blocking layers. InGaN MQW solar cells with AlGaN layers were grown by metalorganic chemical vapor deposition, and high crystal quality was confirmed by high resolution X-ray diffraction measurements. Time-resolved photoluminescence results showed that the carrier lifetime on the solar cells with AlGaN layers increased by more than 40% compared to that on the reference samples, indicating greatly improved carrier collections. The illuminated current-density (J–V) measurements further confirmed that the short-circuit current density (Jsc) of the solar cells also benefited from the AlGaN layer design and increased 46%. At room temperature, the InGaN solar cells with AlGaN layers showed much higher power conversion efficiency (PCE), by up to two-fold, compared to reference devices. At high temperatures, these solar cells with AlGaN layers also delivered super...

Experimental demonstration of non-line-of-sight visible light communication with different reflecting materials using a GaN-based micro-LED and modified IEEE 802.11ac

This paper gives an experimental demonstration of non-line-of-sight (NLOS) visible light communication (VLC) using a single 80 μm gallium nitride (GaN) based micro-light-emitting diode (micro-LED). This device shows a 3-dB electrical-to-optical modulation bandwidth of 92.7 MHz. IEEE 802.11ac modulation scheme with 80 MHz bandwidth, as an entry level of the fifth generation of Wi-Fi, was employed to use the micro-LED bandwidth efficiently. These practical techniques were successfully utilized to achieve a demonstration of line-of-sight (LOS) VLC at a speed of 433 Mbps and a bit error rate (BER) of 10−5 with a free space transmit distance 3.6 m. Besides this, we demonstrated directed NLOS VLC links based on mirror reflections with a data rate of 433 Mbps and a BER of 10−4. For non-directed NLOS VLC using a print paper as the reflection material, 16 QAM, 195 Mbps data rate, and a BER of 10−5 were achieved.This paper gives an experimental demonstration of non-line-of-sight (NLOS) visible light communication (VLC) using a single 80 μm gallium nitride (GaN) based micro-light-emitting diode (micro-LED). This device shows a 3-dB electrical-to-optical modulation bandwidth of 92.7 MHz. IEEE 802.11ac modulation scheme with 80 MHz bandwidth, as an entry level of the fifth generation of Wi-Fi, was employed to use the micro-LED bandwidth efficiently. These practical techniques were successfully utilized to achieve a demonstration of line-of-sight (LOS) VLC at a speed of 433 Mbps and a bit error rate (BER) of 10−5 with a free space transmit distance 3.6 m. Besides this, we demonstrated directed NLOS VLC links based on mirror reflections with a data rate of 433 Mbps and a BER of 10−4. For non-directed NLOS VLC using a print paper as the reflection material, 16 QAM, 195 Mbps data rate, and a BER of 10−5 were achieved.

InGaN-based solar cells for space applications

III-nitrides InGaN solar cells have exhibited many favorable physical properties for space photovoltaic (PV) applications. Here we demonstrate the first nonpolar and semipolar InGaN/GaN multiple-quantum-well (MQW) solar cells grown on nonpolar m-plane and semipolar (2021) plane bulk GaN substrates. The optical properties and PV performance of these InGaN solar cells were systematically studied, including the absorption spectra, current density-voltage (J-V) characteristics, external quantum efficiency (EQE), and internal quantum efficiency (IQE). Overall nonpolar m-plane InGaN/GaN MQW solar cells demonstrated the best performance across all devices, with a high open-circuit voltage and the highest EQE and IQE. This huge difference is attributed to the better carrier transport and collection on nonpolar m-plane devices due to the reduced polarization effects, which were further confirmed by bias-dependent EQE measurements. Furthermore, high temperature measurements reveal that peak EQE values exceeds 80% at 500 °C and a positive thermal power coefficient up to 350 °C for nonpolar m-plane solar cells. These results demonstrate the high potential of nonpolar and semipolar InGaN solar cells for high temperature PV applications, which are required for space missions close to the sun.